Tapping out the cranks??
 

Alright nah.

I have recently picked up an old beat up bike, and it's got some crank issues.
The cranks are threaded (the type where you take off the nut, slap in the crank extractor, ie: square spindle type.) BUT the threads where the extractor goes are sort of scraped/stripped a bit.

I wonder if I can re-tap this thing to make it workable again.
Can anybody tell me what the size/dimensions of tap I would have to buy and if it is likely to come in most conventional tap and die sets?

Most cranks use M22 x 1.0 mm threads. This is a very unusual thread form, pretty much limited to this one application that I've ever seen. The die is very unlikely to be found anywhere but at a bike tool supplier but even Park doesn't list one. Bike Tools Etc offers one but it's $27. Look here:

http://www.biketoolsetc.com/index.cg..._id=MS-M22X1TB

If the threads are stripped, re-tapping won't fix it. You'd need to install repair threads. Even if a repair set is made, though, and you could find someone who has one, the cost would be as much or more than a new crankset.

If you're trying to remove the crank to get the BB out for service, and you can get one arm off, you should be able to take out the BB with the other arm still attached. (Something I've never tried, btw.)

Find another old, beat up bike and take the crank off it. I wouldn't bother trying to fix the threads if they are stripped.

+1. Trying to make the crank extractor work is going to be a lot of work. There are a bunch of methods fro removing cranks with this problem - do a search and you'll find better descriptions than I could give you.