He should be able to cut and thread it by only taking off the magneto flywheel and not disassembling the entire engine, which is what I highly recommend. Be sure it gets cut far enough beyond the magneto to have plenty of threads (but not too many) to engage your new prop hub securely.

(A good way to keep the engine from turning over while you are threading the crankshaft is to put the piston near the bottom of its stroke, and then insert soft cotton rope in through the spark plug hole. Then when the piston goes up, it hits and compresses the rope and the crank stops turning, allowing you to thread the shaft.)

AV8TOR