My AMT600 (circa mid 80's) has a 286cc Kawasaki engine.
The tranny is belt driven from the engine with dual chain drives to each side on the rear.
In the front the width across is: 4 1/2 inches.
In the rear the width across is: 13 inches.
The length from front to rear is: 16 inches.
The height in the back of the seat is: 6 inches.
The foam cushion is: 4 inches thick.
The plywood bottom appears to be: 3/4 inch.
More info to come....