no problem welding them. just get rid of the broken metal and get it all filled down to SMOOTH and then someone will weld a plate in. my advice is to take the cover AND the head and have them bolted together: this stops distortion in the welding process.
you need to check that the cam follower bearings that roll around the cam are still there and working 100%.
if they are and you can account for all the tappet parts then you're OK? but it takes very little effort on the old pre-2001 engines to take the left cover off and check - so why not do it?