I take it this will be a DC machine, Have you checked the carbon brushes are not worn down on the traction motor?

Have you checked all the cable connections to the traction motor are clean & tight?

We sometimes used to get this issue on DC Cesab, it normally turned out to be the motor full of dust / debris & requiring a wash out or in rare cases the motor insulation breaking down.

We used to power wash out the motors to clean them, battery must be disconnected & they need to be fully dried out before use.