I'm converting a riding mower similar to this one with a large, single center blade. Is your mower like this:
https://www.motoruf.com/n/partslists.html?id=644464 ?
If so, the mower design has a separate gearbox driving, through a roller chain, a rear axle/differential . I would recommend you drive this axle/differential with a small electric motor and variable speed controller that replaces the gearbox. The chain drive makes it easy to position the motor in the body of the mower and change the drive ratio with different sized chain sprockets for optimum ground speed. This is a motor separate from the much larger motor needed to drive the cutting blade in the mower deck.
On the mower I'm converting, the large blade drive motor is going to be mounted on the cutting deck, driving the blade through a V-belt. This eliminates the undesirable variable angularity of the V-belt of a typical ICE drive set-up. The V-belt drive allows the motor to be positioned to clear the bodywork as the deck is moved up and down. Also, the blade drive ratio can be changed very easily by using different diameter pulleys and belts. And, the belt will reduce the shock loads on the motor and slips if the blade hits a large object.
It might be possible to have this cutting deck motor run at a fixed, constant speed. This would eliminate the need for an expensive electronic speed controller. Another possibility is to bring the cutting deck motor up to speed in steps to reduce the starting torque shock loads with the use of a large resistor bank, like the old electro-mechanical golf cart controllers. The resistance speed controllers are very inefficient, but they would only be used for few seconds as the motor is brought up to its fixed speed.
