HP = force X speed. What, are they trying to dig deeper? A larger HP motor will give more force, and take more electricity. But it will have the same RPM, output shaft, mount, and pulley. Just be smart, and let your customer know that what you would be limited to doing, would be getting him a larger motor. Whether it solves anything, is something he would be more familiar with...
I'm not sure that it's such an easy answer, because of the control; wiring, fuse size, etc. Make an attempt to find some information on the internet, at least; that old of a unit normally wouldn't have data at the end of a call to a manufacturer customer service department (unless it's a Moore, which would be a contradiction.) But there might be some reference to the original, even some knowledge of what the model number means is helpful. Depending on where it has been installed, there may be a set of third party inspection data. In my jurisdiction, the machine would have a very expensive third party inspection sticker and I wouldn't touch it without first contacting them. Asides from that, the question is 'how do you expect to work on this.'
The motor will have some frame size, which establishes the mount, output shaft, etc. One way to do this is to replace the existing motor with a larger HP unit, at the same time replacing the pulley and belt(s) with identical ones. You can work with Grainger, where they would have a new motor, pulley, belts, and mounting plate. Their catalog has very good pages with motor, pulley and belt information. Grainger is a little more expensive than some, but a 'full line' supplier.
It sounds as if you're not completely sure about this. It would probably be your best bet, to contact someone who can fill in all the questions for you. This would take calling people who rewind motors until you find one who's willing to work with you on this. My guess is that a little help from an experienced motor company will make a big difference towards both finishing the job and getting your customer what he/she wants.