There's lots of guys right here on RCU who will machine you a prop hub for not a lot of money. You just need to know how long you want it and figure out a way to get the right taper on it. Best thing to do--find a guy with a lathe who is willing to make it for $15 or $20 and then sned him the engine so he can measure the taper and get it right.

Don't buy a prop hub from the hobby shop. They will most likely roll their eyes at you because they don't care to support this end of the hobby. They want to sell you a DA or an OS. If you can even find the right size hub--it's gonna cost you a small fortune.

The mount:

Just use longer bolts like you wanted. But, bolt a flat plate of aluminum on the back of the engine. Then drill 4 holes in that aluminum and mount it to the firewall with aluminum stand-offs. You'll probably have to drill a hole in the center of the aluminum mounting plate for the crankshaft to clear.

Typically, you'll put your prop hub on the side that has the magneto. The other side of the engine--where the clutch and sprocket was--is going to be the back of your engine now. That part of the crankshaft will stick out the back and you'll need to drill a hole in the aluminum plate for it to poke through..

You can cut off that side of the crankshaft to shave off some weight and also make the engine shorter. But--and this is very important-- TAKE THE CRANK OUT OF THE ENGINE BEFORE YOU CUT IT.
If you don't--the crank will get hot enough to melt the crankshaft seals on the engine. Just put it in a vise and protect it with a towel or a rag so you don't scratch it. Cut the shaft off with a bandsaw or even a dremel cutting wheel. Clean it up a bit and remove any sharp edges and burrs. Clean all the shavings off. You can dunk it in water--just dry it off. Or use some solvent like Carb & Choke cleaner to blast off the shavings.

Here's a pic of how you might want to make your mount.

Sound off if you need more help.