[sambo]

In my opinion the conversion from cable to hydraulic clutch would be governed more by the engine/gearbox than the fact that it's a Harrison. It might be easier to swap over the clutch for something ligher, otherwise if it's a late-ish model Holden or Ford driveline you should find plenty of conversion options. What's it running?

Pedal box could also be swapped out but it might be easier to get new seats. What are the current seats, not MX-5 I hope? They waste a lot of space in a Harrison.