That's a Mariner 500? I stand corrected. Court says it's a geared motor from FSG.
Price: $1899 | Model Year: 2015 | Powerful geared hub motor and high voltage battery pack for excellent torque and climbing ability Compact folding design with removable battery, comfortable suspension fork and seat post shock Heavier folding design reduces stiffness and can create speed wobble...
electricbikereview.com
Again, any motor will work, as long as it has enough power and speed for you. You might look at this motor/wheel combo. Order it w/o any of the electricals and it's about $150. Shipping is about $150 too. Any motor you buy will have to be installed in a wheel, so it probably less money to buy the combo, unless you're frugal like me. The motor should be plug/play with your cable. I've bought a motor from this guy.
Zen Cart! AKM-128CST 48V500W Rear Driving Hub Motor 36holes with 20/26inch/700CC wheel rim [AKM-128CS 48V500W motor 26/700cc] - ★★★★★ AKM-128CST 48V500W Rear Driving Hub Motor 36holes with 20/26inch/700CC wheel rim Specification: 1. Voltage: 48V 500W 2. Brake type: V-brake, rim brake and disc...
www.topbikekit.com
Being frugal, to justify my ebike hobby, I just buy the motor and put it in a wheel myself, and save the labor at a bike shop.
If you buy a motor.wheel, you will want to pick the higher speed rpm, Typically 260 rpm. Maybe 310 rpm. This is needed to spin the smaller wheel faster. A 210 rpm motor will barely hit 18 mph on 36V.
As for the existing motor, maybe your clutch is rusted up. Spray it with penetrating oil, Your gears are probably freewheel, not cassette. Park Tools has a video on youtube that lets you tell the difference just by looking.