I have an 1989 snapper rear engine rider with an 8 hp briggs. The engine is I feel plenty strong and doesn't smoke or use oil (it's been in the garage unused for about 7-8 years up till a month ago) but gradually over the years it's lost it's speed. I thought it might have an internal clutch going out but don't know.

Thanks in advance for any help.

There is a black clutch wheel on the external area. It is directly underneath the engine and runs against a disc at the bottom of the engine. Check the disc to see if it is worn, cracked, has chunks missing, or is separating. You may have one of two different disc, one with a liner and the other without. Pull off your old one by removing the four nuts, then carry it with you to the mower shop to get a new one.