Can a rubber mounted sportster engine be used on rigid frame?

Yes. This is a common modification done to Harley-Davidson motorcycles, but it does require some frame modification. Rubber-mounted Sportster engines are designed to flex and move with the frame during acceleration and deceleration. A rigid frame does not allow for this movement, so the frame must be modified to allow for the engine's movement. This is typically done by adding gussets or additional bracing to the frame. Additionally, the engine mounts will need to be modified or replaced to accommodate the rigid frame.