Yardman 38" cut, won't drive any more. Any ideas what the broken bit is?

My old yardman MTD 38" cut 12.5 twin cylinder b&s has finally stopped moving. The lead up to this was that you would be dring around and then the thing would slow down in forward momentum.

Then I would press the brake/clutch and stop moving, then release it and it would go forward again etc.

Eventually now nothing happens. It was pushed into the shed and there it has sat for a while now.

So as it is now, you start her up and go to take off and you can just here a spinning noise and thats it.

It has the forward-neutral-reverse transimission with the 8 speeds on the right hand side wheel guard.

Any ideas what might be the issue? Could it simply be a worn belt? or something more sinister?

and the numbers are:
139-685 lot/model
662-E229H mfg date

Peter [u][/u]