Vintage Dirt Bike Q & A

I have a 250 Pursang Mk.4 (mod. 68) that I am hoping to restore. The block is still together, but I'm planning on doing a rebuild on it (bearings, seals, gaskets, etc.). The rod big end bearing is definitely shot, but I'm not sure how feasible/expensive it is to replace it - do I take it to a machine shop or bike shop to have it separated and pressed back together, then aligned? There are several cranks w/rods in decent shape on eBay, but I am not sure which various years/models are interchangeable. The same for the clutch, as the sprocket teeth are shot and i need to replace it.

Thanks for any help or advice!