could you get me a pic of your cylinder? You may not be able to do much with the port timing or angles with the head not being able to come off, BUT, widening and/or raising the exhaust port does show some real gains, but you also lose compression, and since you cant mill that head, i dont know what you can do about that loss, except maybe get a piston with a higher compression height or whatever. You can also port the intake windows, port the hole in the piston, polish the ports, level the piston (make the top of the piston level with the bottom of the ports), epoxy an air ramp in the intake, and set the angles of the lower transfer openings.
You said you had an extra cylinder right? and the head is made on? if i were you, id cut the top of it off and see if i could fab it up to use a gasket and bolts like a regular head (you could drill holes in the fins in order to make "studs" and just run bolts through those holes to hold the head on) Doing this you would be able to rechamber the head (huge gains here) and increase compression.
Has anyone noticed, those KT100 kart motors (regular 2 stroke, 100ccs) make over 16 horses? and thats running with a can, not a pipe, and not alot of mods, you can make way more hp with a pipe. The reason why your motor is rated at 4hp im willing to bet is because of the small detuned carb, and the throttle limiter. Dont you guys think if i had my blaster limited to 4000rpm i wouldnt have any power? Its like this, when im riding, and i get up into the 6-7k range, i hit whats called the "powerband" in a 2 stroke, and once i hit that powerband, the front end is up in the air, and im movin on out.
If the 4 hp were capable of that through gearing alone, and it can be produced at a third of the cost, don't you think the manufacturer would be building riding mowers with 4 hp motors and 8 speed transmissions on them?
Its simple, 2 strokes are being slowly banned in production due to strict manufacturing standards and emissions rules. The reason they dont use them is because at high rpms a 2 strokes oil/gas mixture is being wasted out of the exhaust, and thats horrible for emissions. They dont use them on riding mowers today because in order to make it have enough power youd have to let it rev up higher, which would mean no passing the emissions standards. Another big factor is reliability, 2 strokes aint that reliable, unless you know how to handle them. A 2 stroke is reliable as you make it. Alot of people would be blowing up their mowers if they were 2 strokes. Noone wants to mix gas and implementing an oil mixing pump and other things u need to run a 2 stroke on a mower would cost alot too. So theyre probably doing the best with making 4 strokes.