I know nothing about your tiller but did a few searches. I found several that suggested the model number you listed was also known as a RT5S tiller (rear tine 5 hp?) made in 1985/1986 and I found several ads for manuals for that model.
The Dayco Products Company has an online catalog of lawn and garden belts. A search by model number 51183 listed two belts. L346 Trac Belt L446 Tine Belt
I hope this helps.