What is the service factor on the 1.5 HP motor. The key is to multiply the HP by the SF to get the SFHP, that is the number you want to keep in mind.
Also, are the pumps wired for 115V or 230V?
Options:
1. Replace the motor with equivalent 1 speed.
2. Replace the motor with an equivalent 2 speed ... meaning a 1.5HP
3. Replace motor with smaller 2-speed, but then would also need to replace the impeller. I know there are 1HP 2-speed motors.
4. Replace the entire pump (either go smaller with a 2-speed superflo, or variable, but I think the variable does not make much sense ... UNLESS, you consider changing the plumbing to use the VS for the pool and spa. Although, it may depend on what pump is running through the heater).
1 is the cheapest cost, but will use the most electricity.
2 and 3 are probably pretty similar in initial cost, but 3 would save more power than 2.
4 will cost the most up front, but MIGHT save the difference in electrical savings and MIGHT allow you to get rid of the spa pump.
Although in fairness, you could likely get rid of the spa pump with options 1, 2 or 4. But, the plumbing may get a little odd depending on how you use the heater and how it is plumbed.