If that's a true to size 6 cylinder, this thing should have way more grunt than all 4 banger engines out there, never seen such a big engine in such a small machine until today. Very very rare combination.

I'd try to clean out & clean up the old starter.& Relightly grease up the bearing selves inside of that starter. Then see how it works afterwards for you.

Their neith the cab. I'd check in your manual you got off online for it.