thats a really clean mower!
if you want to make it go faster, dont worry about the engine just yet, get your steering taken care of. bolt or weld the front axle solid, get a direct steering setup made. most everything that is needed to make a fast and safe mower is easy enough with a small wire welder and a little steel.
then fix those brakes, dont use the factory brakes because they will not stop you at higher speeds. get some kind of brake mounted onto the axle so you can stop it.
and then, once you gotten it safe, make the changes to make it faster. but we aware, that leaving it at stock height will make it more likely to tip over. so i suggest you aim for a gear ration something like 9:1. that is the old IMOW ratio, it will give you speed you want, BUT ONLY do this after you have done what ive said about the steering and the brakes.
it is a matter of safety first, no one on this forum wishes to see anyone get hurt. by just making the mower go faster, without making the changes said, there is a HIGH risk of being injured, and with the right situation, killed.
-Phil