A lot of power tools were sold with the motor separately so it can use what you have or can find for a reasonable price. I'd put a 1750 RPM 1/2HP on it. You can ask some woodsmiths what ratio but I'd use a bigger pulley on the motor than what you find on the jointer drum. You need to spin the cutter head to about 4500 RPM. That was what was in the manual for that size 2 bladed machines.
Here's a calculator.
Also, regarding the post by
FrankLee, it depends a lot on what you are going to do with it. Edge jointing doesn't need much power.