Sweet 8) Looks like a `78 (coaster brake tab) and in awesome shape too. Get you a plastic tote just big enough to lay frame and fork in sideways and cover it with an oxalic acid solution (wood bleach), or white vinegar or maybe even lemon juice (yes, lemon juice because of the high acid content). The decals will not be harmed but the nickel finish will come shining through.
What is the serial numbers on it? They will be on the BB shell. Here`s an example of what a `78 serial would look like:
CC8
365493
The first "C" is for Chatsworth. The second letter which in this case happens to be a "C" indicates month of the build, 3rd letter 3rd month. Third character which is a number in this case is the year of build. They used this format from `77 (maybe even some in `76 and or `75 but highly doubtful) until mid to late `81. If its a Team or Super there`ll be a "T" either above the CC8 or preceding it with a "-" (T-CC8)
The old Mongoose Motomag is by and far my fave of the BMX`s. And with this baby being a survivor, wow. Do yourself a huge favor by keeping it, every part of it. From the box bars to the Ashtabula stem and cranks to the snowflake chainring to the seat to the seat post clamp to the pedals to the Motomags. That is a beautiful bike and you will kick yourself if you part ways with it, I promise ya lol, I know, I am still doing that lol.