I suspect either the belt is slipping or shear pins or a hub key is sheared.

Welcome aboard. I suspect you have a Jacobsen built unit, they were good tractors. Can you post some pictures?

A photo would be helpful. Most of the front tine tillers of the 70's era used a worm gear drive. On the Gilson these were a problem. Simplicity Roticul also used this system but it was more durable. Bottom line. If the gear box is tight, then it may worth sticking a new or used engine on it. They used double pulleys and two belts. Forward belt is simply put directly on the inner pulleys directly from engine to drive unit. An idler pulley serves as the clutch. The reverse belt is twisted so that the reverse pulley turns in the opposite direction of the engine. It is controled by an idler pulley attached to the reverse lever. Each works by tightening the respective belt. Pretty simple system you should have no trouble with it. Main point tho is to make sure that the gear box is in good shape.

I found this on E-HOW. ============================= Tips & Warnings If one of the spark plug sockets is empty, then your spark plug was forced from your engine during the blow out. In this case, replacing the spark plug and wire is mandatory. ========================= Really ?? !! I wondered about that but this removes all doubt.