There was a thread a few posts down which explained how to do this but the pictures have disappeared from it making it pretty useless as a guide.
To do the re-stringing you need some elastic (I use the same circular elastic to do this as I fix blue pant figure neck posts back on with, probably 4mm or 5mm diameter).
I don't cut or remove the leg rivets, I cut through the old eyelet that held the old elastic, I then cut an electrical 6mm yellow ring crimp, thread it round the old rivet and close it up again.
You don't need to split the body to re elastic it, just cut the old elastic and retain the metal hooks.
A good guide to this can be found over on Unofficial Action Man HQ forum (I am sure Dal won't mind me referring you over there are Rob from there is on here too), where there is a very extensive sub section on repairs dating back a good 10 years with lots of useful info.