My old MTB with new fenders
Schwinn bikes were made strong but they had their own standard sizes for everything that makes it hard to find part now.
The Varsity, Continental, Suburban (upright bar Varsity) all have 27"x1-1/4" wheels, which you can still find parts for.
The stems and seat posts are a goofy size, but Wald still makes a Schwinn size seat post for these more popular Schwinns.
Also, a trick I learned is to insert the seat post upside down so there is no tapered end to deal with.
I think old Schwinn bikes could really use a Cyclone motor, because big motors will not fit them and it may be impossible to upgrade to hill climbing gears.