I use MEK as a plastic welder too. It has been off the shelves here in California for a couple of years now so I'm glad I bought some when I did. I find it to be the best, as it melts styrene quickly but doesn't evaporate as quickly as acetone for example.
I have used acetone with some success, although I notice it works well on the styrene of some kits, while not so well on other kits. I'm guessing there must be a variation on styrene formulations. It is certainly not as aggressive as MEK.
I have recently bought a can of the MEK substitute but haven't tried it yet.