radiator drain plug leak

[wojtek_f]

Hi
I have '93 SL1 manual.
About a week ago I started to loose coolant. This only happens when the car is driven (level doesn't noticeably go down overnight).
Yesterday after an hour of car being parked I crawled underneath and the leak is clearly coming from radiator drain plug. I didn't try to do anything than because it still might have been hot. The radiator was replaced in the Spring last year so it should be still fine but of course out of warranty (is there a self-destruct timer on those things?). It's an aftermarket piece of work put in by a shop near where I live. I don't know the brand of this radiator.
Today I tried to tighten the drain plug but it just keeps turning - it should just turn 3-4 turns before being locked right? It started to leak when I started tightening it. I can turn the plug with my fingers through the hole in the plastic shield. I think I turned it like 10 turns and doesn't seem to get any tighter though there is resistance when I do this. In the process I made a 1/2 quart antifreeze mess in the driveway. My guess is that the shop I did the repair last year overtorqued the cap and stripped the plastic thread on the radiator or the plug and let me drive out with this. It somehow held on for a year and now it had enough. I think it's 'skipping' the thread on the last turn.
What can I do? I don't want to have to spring another $400 to have the radiator replaced. Can I buy a radiator plug separately? (Are they all standard size for this model even for aftermarket rad's? Can I add an O-ring (or two) ? What size? Teflon tape? The radiator part that the plug screws into looks to be made of plastic (I might be wrong it's pretty tight under there without jacks and I don't want to be directly underneath ethylene glycol leak to take a close look) so I hope the thread on that is not totally stripped.
Any ideas?
thanks,
Wojtek