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Assumeing it is a twin cylinder, in theory a header might marginally help it, but I SERIOUSLY DOUBT IT. If it is a single cylinder, cant have a header.
Try some motorcycle high performance exhaust.
If you were going to hot rod the engine for lawn mower raceing, I could see it. If hot rodding for grass cutting, (sustained over RPM , since you removed Governor) its going to blow.
Round-Up the grass and call it a day! All jokes aside I believe above a certain speed you will be tearing at the grass and not cutting it with the blades. I agree either you have speed or cut grass. I note those folks who race have dropped the cutting unit off.
just find you an old powerstroke and stick that baby in there. really get those blades spinning. (Better plan on beefing up the front suspension though)
if it has a screw in muffler you could use natural gas pipe(seen it). if the muffler has a flange then you would have to weld a thick washer to the pipe with the old flange behind it. you need to make the pipe at least a foot long so you don't burn a valve. my friend use to race go-carts and they would use pipe for a few of their carts.