On a delta-style trike, you're gonna have non-standard parts due to the rear end. On most 2-wheel Schwinns, you got 2 parts that'll be weird in terms of interchangeability. (3, if you count the headset, but most every old bike manufacturer had weird sized headsets.... and Schwinns were about as close as you get to a "standard" as old American bike headsets get. Most cases, some fresh beafrings and you're good; those old Schwinn-approved headsets were hard as can be; they usually just need a cleaning, new grease, and fresh bearings. The cups, cones, and hardware are really tough, and the chrome usually comes up real good with a little bit of elbow grease and magic. Try diet coke and tinfoil....)
Here's the two oddball parts:
-Crank/BB. Schwinn one-piecers ran 28tpi hardware; standard is 24tpi. Sp, the WallyWorld BB won't fit the Schwinn crank, and the WallyWorld crank won't work with the Schwinn BB. So, the trick is, if you have to replace the original Schwinn cranks, just use BOTH the crank and the BB from the WallyWorld donor bike.
-Tires/rims. As discussed above, Schwinn ran some weird sizes, with a lot of the ubiquitous cantilever frames running s7 kit. So, same deal as the crank-- if you gotta swap in new tires or new wheels, just take BOTH the wheelset and the tubes/tires from the WallyWorld donor bike.
The rest of the stuff that doesn't always interchange is normal stuff, not proprietary stuff. Depending on the year, a ChiTown Schwinn might take a 7/8" stem, but more often on 60s, 70s, and early 80s Schwinns, you'll need 21.1mm stems. This is true of most all American bikes from that same time period. The seatpost is 13/16th, which is weird enough, but they're EVERYwhere and usually free. And, to be fair, a lot of other manufacturers ran weird sized seatposts. Seatclamps are kind of tricky, b/c like most bikes of the style/vintage, they take a regular 1" ID clamp but i've found that some 1" clamps that were made for bikes with 7/8" seatposts, you won't be able to tighten'm down enough on a Chicago Schwinn. Usually, primitive-looking clamps are better, and pimp-looking bmx clamps are less likely to have enough "bite" for a Schwinn. Oh, and a lot of Schwinn seats/seatposts are set up for 5/8" guts, but modern guts are 7/8".... this isn't a Schwinn problem, though; it's a general old-bikes problem...
HTH. And really, try out a nice 26" Schwinn. Canti frames are still all over the place for cheap. PErsonally, I think the BB is kind of low for my preferences offroad, but they make incredibly nice-riding around-town cruisers.