I have loads of old magnesium minilites but they need a lot of TLC - I approached Minilite about having them looked at but when I told them they were original ones rather than the magnesium versions that Minilite are making now, they didn't want to know - perhaps they'll only service new(ish) ones?
There is a bloke in York who by all accounts is completely fearless and welds up magnesium wheel with his teeth - I spoke to him about mine and he didn't see any problems at all but said it wouldn't be cheap
here's his website
http://www.ianjemison.co.uk/
Having said that my uncle is a metallurgist and he really didn't like the sound welding magnesium due the the effect on the surrounding metal.
These wheels are just so light it's ridiculous but they've been sitting in my garage for ages without protection and I doubt whether they're 13" in diameter any more...
Personally speaking I'd only go for new ones. They are widely used but professional teams life items such as these.