If you would consider staying with the rover engine, this company:
http://www.automotivecomp.com/landrover_perf.html
Do a number of tuning upgrades for 2.25/2.5 Series Land Rover engines. exchange rate and freight would probably make it too expensive however.
I had one of their Stage 2 gas flowed high compression heads a long time ago on my S2a. With their head, carb and cam they claim an extra 43bhp.
They even do engines up to 2.8l.
As another option my youngest brother has fitted a supercharger off a Mini Cooper, with a different front pulley to suit on his s2a
Seems promising....

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