If you got to either of the antique Cat sites there is info on converting to direct electric start. I'd think that'd be far, far easier than cobbing another engine to the diesel. Fot that matter you'd even be better off belting another 10 or 15 hp engine to the pony flywheel and going through the pony gearing system.
Myself, ponies aren't that hard to find in running condition for a reasonable price. I'd find one and exchange engines. I promise that will be more likely to succeed than cobbing another engine onto the diesel.