There are several power meters that will work on a Cannondale MTB crankset:
Spider Power Meters
power2max NGeco Cannondale MTB Power Meter
The power2max NGeco Cannondale MTB Power Meter is one of our go to power meters for Cannondale MTB cranksets. It’s a superb power meter in that it’s accurate, extremely durable, measures the power from both legs, is reasonably priced and allows you to keep your current Cannondale crankset.
The power2max NGeco Cannondale MTB Power Meter is compatible with Cannondale SI, SISL or SISL2 cranksets, as well as the Cannondale ONE. Please note, the power2max uses a 104 BCD chainring configuration. So you will need a power2max compatible MTB chainring as well.

power2max NGeco Cannondale MTB Power Meter
XCADEY XPOWER-S GEN2 Cannondale MTB Power Meter
The XCADEY Cannondale MTB power meter is very similar to the road unit above, however it is for MTB so it works with 104 BCD 4-bolt MTB chainrings.

XCADEY XPOWER-S GEN2 Cannondale MTB Power Meter
Pros: Moderately priced, don’t have to change crankset, measures both legs
Cons: None
Power Meter Pedals
MTB power meter pedals are also an option as they can easily be installed on any MTB crankset. We have the Garmin Rally XC, SRM X-Power and SRM X-Power Flat Pedals.
Pros: Easy to install, works on any bike, can move from bike to bike, don’t have to change crankset
Cons: Some versions are pricey