I would build a guided spring helve hammer if it were me. The problem with the rusty design is that there is no intermediate flywheel.  The slack belt clutch works directly between the motor and the pitman shaft. This leads to slow jerky starts and excess belt wear. I solved this issue by installing an intermediate jackshaft with flywheels. The motor spins the jackshaft with 2 tight vee belts. Then a flat belt pulley in the middlde of the jackshaft transfers the power through