rebuilt mine a few years ago after it ran a big end bearing.
i replaced with new:
crank
cam
followers
rings
oil pump
oil pump drive shaft
shells
These were the things that were most worn out the rest were still useable. i had rebuilt the head not that long before so it was still good.
The whole job didnt cost a massive amount and only took me a few days and it was back in running well.
its well worth doing as you then know you have a damn good engine.
The only thing i had trouble with was the gaskets. buy a genuine gasket set as cheap ones are really ****e and generally you want oil and water to stay in your engine!