It looks like your trying to build a safe machine! What you are building would be a GP under the USLMRA rules. Single cylinder governed at 3650 RPM's.
You have a good start.
But have a way to go yet.
First you need safety teather. So when you fall off the mower shuts down.
You need a different front axle. The stocker WILL brake at any speed over stock. You WILL get hurt when it brakes.
You need better brakes. The stock brakes won't stop the mower at speeds over stock for long. When they fail you WILL get hurt.
You need to upgrade the steering and the linkage. It WILL fall apart at speeds faster than stock.
Good start! Keep going!!