Best new motor for an old procat?

[AndyF]

I have a procat which I dug up last year from my loft. I want to get it going again with a view to messing around with it but also maybe a vintage race.

I want a motor that will give it a better turn of speed than a std 540 motor, but not some new era monster, that will trash the transmission and flatten my cells too quick. With a view to being legal and competitive in vintage comp too.

Oh. Don't want to spend much either!