Chainsaw Repair

Yes on the clamshells.

Stihl MS250 Chinese clone ($32.50 w free ship) would fit the bill. You would need a hub, electronic ignition ($50), and a intake manifold/carb.  Could even use a MS 250 muffler especially if you get the two piece one that comes apart so you can open it up some. Saw off the PTO side of the crank and your prop hub goes on the flywheel end

there are people around that can machine you an 6061 aluminum hub that threads onto the crank which includes the magnet for the ignition timing sensor. These same people could make you the aluminum carb adapter.

A Walbro WT215 carb from Stihl is $22.50.

It will turn a 20 X 8 prop around 7,000 rpm which is ideal.

The typical scenario is to have a aluminum cylindrical adapter with set screws impinging upon the existing carburetor spigot. You with then need a white nylon or black Delrin insulator between the adapter and the carburetor or you will have hot start problems.

If you commission someone who has made the timing/prop hub and carb adapters before, they will know exactly what you are talking about. 

I have a 50cc Echo/Kioritz 621 already converted if you are interested. it turns a 22 X 8 at 6900 - 7000. It was sold as a BME 50

These pictures are stock pictures