I did loads of mod's on mine.
It had an 1100cc short stroke engine, which I bored out to 1160 to fit a 1300cc head, stage 2 cam, double valve springs, split manifold, twin 1 1/4" SU's, oil cooler, and a full RR tune with carb needles profiled to the cam... then I did the brakes which were originally drums all round. Perhaps should have done those first in hindsight, but with mini-fins on the rear and 8" Cooper disks on the front (no servo) is was much better.
Absolutely RAPID up to 65mph, but topped out at around 85mph due to the final gear ratio. It was great fun absolutely destroying XR2/XR3i's off the line at traffic lights.