OK, for 150-200 decent engine, 150-200 decent bike at Walmart, get a mongoose if possible since they seem to be good for customer service (you'll want it trust me, and comes with the bike no extra charge, they once sent me a brake caliper when it broke unusually soon, and a rim that had a spoke pull through they replaced with a whole laced front wheel minus the tire and tube) and you can depend on the frame if it's steel.
My bike came from the local wal to wal hellmart and it had a small tear in the seat and the handle bar was broken, they were going to send it back but I caught them in time and talked to the manager, he marked it down from like 215 to I think 165 or 170, it was a good deal, I bought it.
Another bike I bought was a huffy fortress. It has those big tires, and it takes a standard kit really rather well, and holds up to a man definitely over 250. I bought it for I think 58 bucks or something ridiculously low since it was a return, whoever bought it didn't want it so they brought it back, and it got marked down simple as that.
They apparently don't have very bad customer service on bicycle parts.
Either way you'd be getting the same exact thing, but possibly at a better price and even better get something possibly better or more suited for your size.
The mongoose terrex I'm using is a steel frame, very strong by my experience, and can be fitted with a standard motor rather nicely, especially if you aren't using a jackshaft.
The disk brake on the back wheel can be removed, and a sprocket put on that bolts to that 6 bolt pattern.
Just some options that can help liberate you from the gasbike/bikeberry/other-crap-hole ongoing circus of a business.
