You will have difficulty making a decent motorized bicycle at $250 but it's possible. You will have to buy a used bicycle for sure. Sometimes you can get a good cheap bike on Craigslist or similar site. Look for steel frames with a large enough V in the middle to mount the engine. Also, look closely at the rear wheel, you want wide rims so you can put fat tires on them. You want it to have 36 spokes so you can mount the rear sprocket that is driven by the engine, you can use either a rag joint set up or a sprocket adapter set up to mount the sprocket. Sprocket adapters are better because they drive the hub and not the spokes. All mountain bike rear wheels have a hub with many sprockets to change gears on the peddle side, try to find one with the largest diameter hub you can if you want to use a sprocket adapter. I use hubs that are about 2" in diameter on my mountain bike so that that the sprocket adapter that I use will fit. I found a great deal on an older mountain bike on craigslist for $10 once, it's possible.