Raise the ports and so the port timing.
Raising the exhaust will give more RPM and raising the intake will give more power but creates also also more consumption.

Raising the sleeve by adding shims is an easy trick which can also be undone. But it is depending the amount of headshims you have, if currently there is only one 0.1mm headshim there is no room to add shims under the sleeve because what you add under the sleeve you must take away from above the sleeve to keep the combustion chamber on the same level. It is wise to buy a shim set because you have to modify one to fit it arround the sleeve.