I rebuit the drive on a Troy self-propelled. It's basically a variable speed friction drive. The crunching sounds like the bearings on the drive shaft. You will probably have to replace the bearings, friction wheel and belt.
The drive system is a little tricky, especially to put the belt back on. It does require some torx screwdrivers. To get the tension right, you may need a feeler gage. 0.040 might be the right clearance when non-engaged.
If the wheels are worn, they will have to be replaced too. If the parts are not available through
MTD Products Landing Page, you will have to resort to a lawn mower shop with aftermarket resources.
You could have said a current non-self-propelled self-propelled mower.
If the bearings are shot it makes sense to get the repair done ASAP before it does shaft damage,
Take the plastic plate off on the front of the engine, to assess if it's beyond you.
Small engines are cake compared to a vehicle carb rebuild with 120 parts or a steering rack replacement or removing the intake manifold of a foreign car with a carb.