Active 20 is the same as the S200 so any motor assembly listed for either of those two would work. I would suggest you buy an OEM part only from a Dolphin dealer … eBay and other sites that list them cheaply are probably just scrap motors pulled from other robots. Current prices for the replacement motor assembly are $450.
At that prices, your about half the cost of a new robot. Makes more sense to buy new as I guarantee something else will fail next and you’ll just winding dumping a lot of money into a money pit of repairs. However, if the robot is only a few years old, you may get away with motor replacement only.
My 5 year old S300i died and I tried to repair. Couldn’t do it as Dolphin limits certain critical internal parts to the motor assembly. So I decided to go back to using suction side cleaners because I didn’t want to pay the cost of a new robot every 5 years … YMMV.