John,
No, I've never uprated a lathe motor but many people do.
Just look at the number of 3 phase inverter controlled adaptors available for the likes of Graduates etc., but finding a small enough/strong enough motor might. I think the likes of The Brush Motor Co. are long gone, nowadays it's all standard fit mass produced motors. So you might be looking at a new mounting arrangement to hold a larger motor.
As long as it's done properly by a qualified person (look up Motor Rewinders in the Yellow Pages) and PAT Tested don't think it will be much of an insurance problem.