All of my engine modifications are aimed at improving low and midrange torque between 2,500 - 4,000 rpm.
The best single item is a reed valve intake, and Rock Solid Engines manufactures the best reed valve intake for standard engines.
The next best thing is a Jaguar CDI which lets you reduce the ignition curve to prevent detonation when fitting a higher compression cylinder head. The cylinder head on my bike is the awesome (medium compression) twin spark plug CR Machine Manufacturing billet cylinder head, as it has the greatest cooling surface area of any head i've come across, and it's available in both medium and high compression versions.
The other item on the list is a diaphragm carburettor, which allows adjustment of the low and high speed circuits with a simple twist of a screw driver. No more messing around with jets and spilling fuel everywhere taking the carburettor apart.
I've tried all of the float style carbs (genuine Dellorto, clone Dellorto as well as the standard NT) and went back to the standard NT carburettor, that is until the diaphragm carby was fitted. I've never looked back after fitting the diaphragm carburettor, and the thing doesn't leak fuel as all the float style carby's seem to do.