Note:
1: Amperage 
This is your electric motors TRUE power rating. HP ratings have been made suspect over the years by large retailers who like to use "peak HP" or " SPL " in their terminology. The Motor's amp rating does not lie. Please make sure your FL amp (FULL LOAD) rating on the motor you're replacing is within 10% of the FL Amp rating of the motor you intend to buy.
Example 1: If your current motor says  21 FL amps at 230VAC. Then you would be safe replacing your motor with a 19FL amp thru 23 FL amp electric motor 230volt.
Example 2: If your current motor says 21 FL amps at 230VAC. Then you replace it with a 15 FL amp motor. That 15A motor is NOT strong enough for the task, and will indefinitely fail.

2: Frame Size 
The frame not only constitutes the bolt pattern, it also depicts the overall size and torque. The larger the frame, the stronger the motor. If your current motor you're replacing shows 56, 143T or 145T on the nameplate. Its extremely important that you replace it with that frame size.  Improperly sized motors will inevitably result in motor failures.
Example: If your existing motor says 5HP 184T frame, and you elect to replace it with a 5HP 56frame.. That would be similar to using a 4 cylinder engine to power a school bus.  This is why its important that your new motors frame size matches the old motor you're replacing. This will ensure your replacement motor will work for years to come.

3: Phase and Voltage
If your machine requires a 230volt electric motor on single phase power. You would want to incorporate a 1-phase 230vac electric motor for your application. 230V electric motors cannot handle voltages less than 208V. They will run too hot and you will experience motor failure.

4: RPM
Matching up the RPM is quite important. The wrong speed can damage your pump.  The RPM does not have to match exactly, but you do need to be the neighborhood.
Example: 3510rpm, 3550, 3450 are all the same. They all fall under the umbrella of whats known as 3600RPM motors.

5: Shaft Diameter
With a tape measure. Measure the outside diameter on your electric motors shaft. By matching up the proper shaft diameter, you will be able to reuse the original motor pulley that came on your machine.