Welcome aboard! Here's a thirteen page thread devoted to customized Cranbrooks:
http://www.ratrodbikes.com/forum/index.php?threads/show-off-your-finished-huffy-cranbrook.32667/
As far as what will fit, there's going to be a learning curve -- luckily, this site should show you most of the things to look for! Then again, we may not know that you may already know something, so ask questions if we skip a step and/or pardon us if we over-simplify something.
Pedals only really come in two sizes: 1/2" thread for one-piece cranks (like the Cranbrook) and 9/16" thread for three-piece cranks. There's a huge selection of BOTH sizes, though, so as long as it says "1/2"" they should fit your bike.
Older-style threaded headsets like on the Cranbrook are usually going to be 1" diameter, but if you want to replace the stem the piece that it fits into (the steerer tube) may be either 21.1mm or 22.2mm in diameter -- which should be printed on the current stem. If you want to replace the handlebars (with something normal and off-the-shelf) those are usually going to be either 1" or 7/8" diameter, and in a perfect world you'll find a stem to fit both the handlebar and steerer tube. Since that works out to "only" four different stem sizes, it's fairly easy to find one that works. (1" x 22.2, 7/8 x 22.2, 1" x 21.1, and 7/8 x 21.1. Some websites will also confuse things by putting the first measurement in metric as well, so those four sizes become 25.4 x 22.2, 22.2 x 22.2, 25.4 x 21.1 and 22.2 x 21.1.)
Edit: I see you mentioned "springer forks," which opens up a lot more questions. The Cranbrook comes with a 1" threaded fork and headset, which means that most 1" threaded forks will fit. Threadless forks are different beasts altogether, and use different stems as well -- if you WANT to go that route it's possible, but it's going to be more complicated and I suspect more expensive. Do you want to keep the same 26" wheel on the front? If so, then you have to decide if you want a straight fork, or one of the "slightly bent" ones with a curve in it, or one of the extremely bent ones as often seen on "low rider" bikes. Then again, those are all the "vintage" style forks -- for all we know you might want to put a more modern motorcycle-style fork on there! Lots of choices, lots of decisions.
If you want fenders, you'll probably want the wider style fenders made for 26" cruisers. If you want different tires, you'll want the relatively common (for instance) 26x1.95 tire -- something with a
decimal number after the 26. (Older cruisers from before the era of mountain bikes used a DIFFERENT SIZE of 26" tire, with fractions for sizes like 26x1 3/8, but you'll be better off using the decimal sizes AND have a much wider selection of sizes and styles.) Wider tires may or may not fit the bicycle frame depending on the width, but there's probably a lot more information on that subject in that thread I linked above.
Seats are pretty simple -- most cruisers have three pieces: the seat, the clamp, and the seatpost. Seatposts come in all sorts of different diameters depending on the company that made the frame, but if you need to find a replacement that should be easy enough. Most cruisers today probably use a 7/8" clamp, so many seatposts will be listed as "XXmm, 7/8" clamp end" which means the post narrows at the end so the clamp fits.
As far as seats -- wow. On this forum alone there's probably been enough written to fill several books.
Some people love the vintage seats made with real leather and springs, some like the newer ones with newer materials. There's no such thing as "the perfect seat" since it depends on the rider's anatomy, their "sit bones," and their style of riding. People on cruisers tend to sit more upright than people on racing-style bikes, so a wider saddle on a cruiser will probably be more comfortable since it's supporting more of their weight. On the other hand, the wider nose of a cruiser saddle may become uncomfortable if someone's going to be riding 50 miles in a day, but that's also why they make road bikes and narrow saddles, which are a more efficient way of covering long distances. The best thing I can recommend there is to try the one that came on the bike, and if you don't like it, try to analyze WHAT you don't like and then research seats around that. (Then again, this spring I just put a banana seat, "sissy bar" and "ape hanger" handlebars on my latest bike -- essentially making it fit and feel like an adult-sized, old-style Stingray -- so you might not want to trust my advice when it comes to seats.
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Some purists and vintage bike lovers will howl at my over-simplifications above -- I know there are errors and omissions because there's ALWAYS going to be some weird European size for everything bicycle-related -- but the above is probably correct for 99% of all cruiser bikes made after 1970 (and probably correct for MOST "department store bikes" in the same time period.) If you can't find something in the right size, rat rodders can always find a way to MAKE something fit -- like with shims -- but I'm not a huge fan of those on handlebars if the right size can be found.
As I said above, there are lots of choices and decisions to be made with any of these customizing options -- depending on your point of view that can be frustratingly maddening or exhilaratingly liberating! These bikes can be like a blank canvas, which can be turned into almost anything, limited only by your mechanical skill and knowledge.
Enjoy yourself, ask questions, and have fun!