I have a 2017 Massimo MSU500 side by side. It's not really that hard I have changed mine a couple of times to include the clutches primary and secondary also the wet clutch.
To my guess form what you're saying it sounds like the riv limiter. That is protecting the motor. I have a 2017 Massimo MSU500 and when I replaced the ECU you have a procedures that you use to reset it and you increase the rpm to set it which is about 3 to 4 hundred rpm
On the massimo msu500 , Start the motor, then press on the gas all the way and let the riv limiter cycle holding the gas pedal all the way down, turn off the ignition, then let up on the gas pedal. Then it should be reset.