Troy bilt tiller engine

post a model number for better answers.

Are you sure that the original engine was a Briggs? Or was it a Tecumseh? For tiller and snowblower applications, Tecumseh offered engines with counter rotating camshafts, and mounted a pulley on that as well as the crankshaft. This way, on your tiller for example, all that was needed was belt and tensioner on each pulley, driving it forward from the crankshaft and rearward from the camshaft. The obvious benefit for manufacturers at that time was that they did not need to incorporate a reverse in the transmission because it could be accommodated by simply adding that second belt and tensioner. The downside today though is that, as far as I know, Tecumseh was the only manufacturer to produce this style engine and they are no longer in business.

At Dexter said about the Tecumseh engines is what I have found also. You may want to look at a new Troy built like yours and see how they handle the forward reverse. I'm guessing they have to put a transmission of some kind in but maybe another way to do it. Certainly worth looking at a new one to see what they are doing.