I'll start off with this: The way you drew your plan... The motor is a lot bigger in person than how you drew it. I know what it's like to try crazy ideas because you don't have the money/tools/space/whatever. I've been there.
Many people have used that engine before, but never as a friction drive, because it's way too big and heavy. I suggest that if you do insist on using that engine, do yourself a favor and put it inside the frame. It's much more secure there, plus the weight would be more even on both wheels.
Also.... Aluminum is not a good idea for that big of an engine. Especially aluminum bikes that you'd find in Walmart. If you ever hit a big bump or a pothole, your frame can bend or snap and it would not be pretty.
If you have a Goodwill or some other thrift shop near you, you may be able to score a good bike with a steel frame for pretty cheap. As for the drivetrain parts, put your cash onto a pre-paid debit card you can get at a Walmart, CVS, Walgreens, etc. and you can use that to get a throttle assembly, a centrifugal clutch, a sprocket for your back wheel, and a chain.
If you have a local metal shop, you can find yourself a strong sheet of scrap metal that you can use as a mounting plate. Drill some holes to match the mounting holes on the engine. If you're lucky, you could ask the shop where you can find someone that could help weld it to your bike frame. Good luck with whatever path you choose!