The existing recepticale is a 20 A 125V Hubbell twist lock,
The plug on the pump motor is 15 A 120V.
What is the best and proper way make them compatible.

The existing recepticale is a 20 A 125V Hubbell twist lock,
The plug on the pump motor is 15 A 120V.
What is the best and proper way make them compatable.

Is there any other receptacle available? If the pump has the original cord and they are rated for 15 amps, then you don't need a 20 amp circuit. The 20 A receptacle has different shape so you won't plug a 20 amp device into a 15 amp circuit. It's OK to plug a 15 amp device into an outlet protected by a 20 amp breaker.

If for whatever reason you want to use your Hubbel 20 amp receptacle, you'll have to change your plug to a Hubbel L5-20 or L6-20 depending on whether your receptacle is L5-20 or L6-20.

Also make sure that isn't a 240 volt receptacle. There is a L14-20P plug for that. But you wouldn't want to use that with a 120 volt pump.

What you're considering doing would actually create a dangerous situation, possibly leading to a fire.

The intent of a circuit breaker (hear me out here, this is leading somewhere relevant) is to protect the wires running from it, to the "appliance" from over heating and catching fire... sounds weird, most think the breaker is intended to protect the "appliance" or what's plugged in at the end, but I'll give a quick explanation. Most normally think the breaker is to protect the appliance, in this case the spa. Lets say you have a 240v spa designed to use a 50a circuit. Say the pump is designed to use 10a, the heater uses 25a, there's a blower that uses 8a... whatever. The spa takes a 50a breaker to operate. The wiring running from the breaker to the spa has to be able to handle 50a of current running through them. Pretty obvious, right? Ok, so what happens if the pump, that's only rated for 10a, malfunctions and starts pulling 30a? The 50a breaker won't trip... but the pump is going to be totally fried... the main circuit breaker will NOT protect against something like this.

So, what the heck does any of this have to do with your question :-)?. You have a spa with a cord, with a plug on the end, that's designed to go into an outlet that will supply a maximum of 15a of current. You want to change the plug, so you can plug it into a outlet capable of supplying 20a of current. "Sounds" like a no brainer. Problem, the CORD of the spa, is probably only capable of carrying 15a... IF the spa malfunctions, and tried to pull 20a, the breaker won't trip, and the cord could overheat and start a fire.