Unless your electricity rates are quite high, you will never recover the additional cost of a variable speed. I don't think you can replace a single speed motor with a variable speed motor. I think you would have to replace the whole pump, and that can be quite expensive. You probably will save money in the long run by getting a two speed motor. Or you could get a standard or energy efficient single speed motor. The two speed motors will use about 1/4 of the energy when it is on low speed of what it uses on high speed. So, if you don't increase the pump run time, you save about 75% of the energy cost. If you get an energy efficient single speed motor, you save on the order of 10-15% of the energy from what a standard single speed motor uses.
What BoDarville says about the Service Factor is correct. Your existing motor is a 1.5 SFHP motor. The two speed equivalent motor to the UST1152 is a STS1152R, based on the model numbers (But it is a 1.5 hp with SF=1.3, so the SFHP=1.95, so it is really a more "powerful" motor. It is okay to use a motor with a higher SFHP without changing the impeller.). A STS1102RV1 would also work because although it is 1 hp, it has a service factor of 1.5, so the SFHP is 1.5.
Here is one site that sells those motors.
You can use a two speed with just a regular timer that you use with a single speed, but you will need a double throw switch to change from low to high speed.
If your motor won't start but just sort of hums, the problem could be a capacitor, and it may make sense to just replace that.