I’ll take a look in the basement at the shop. We just donated a bunch of bikes to a non profit in Hartford, so we might have slim pickings. There’s usually something down there, though. The Trek will do just fine. I’d just make sure the brakes work properly.
The old rigid bikes are the best for this kind of project, so your old DB will do just fine.
I’ve worked on quite a few of these converted bikes. Unfortunately we have to turn most of them away, as we don’t have insurance to work on anything motorized. We’re also not set up to deal with gasoline spills etc.
Thengine puts a lot of extra stress on the bikes, and keep in mind these things weren’t meant to have engines, so the parts on them reflect that. They are a lot of fun, though, and like with everything else in the world; if you use common sense you’ll be fine.