sorry mate, can't really help, but I have the same problem with my startrite. It is underpowered and I want to 'soup it up'!
I have looked at machinemart, and what I learnt was that I require 4 pole (or is it 2) and at least 1hp. However I have also been told to double what I think are my requirements and I'll be close.
I will be very interested to see what you decide to do, as I have been warned off MM, having been told that English motors are far superior, and would be a far better bet. Maybe look for a 2nd hand motor? Ebay maybe, but again, be careful as you don't want to buy a duff motor.
Sorry I can't be more help, but if I do get any result for mine I will PM you and let you know. What hp do you need?
HTH
Neil