Souping up an O-360?
Not fuel injected but the mod is too expensive to consider in the UK.
K&N filter, LASAR ignition may be possible off the back of current approvals and STCs - worth a couple of percent. The powerflow exhaust isn't yet approved for the 181 just 180 under s/n 1761, but it would be the simple best improvement judging by its effect on the other aircraft the system is fitted to.
Knots2U are probably the best bet.