Points system details are here:
https://www.gov.uk/vehicle-registrat...tered-vehicles
Looking at it, if you keep the suspension,body and steering original you have a bit of leeway with engine,gearbox and and axles, so long as you don't alter the gearbox tunnel to accommodate a bigger gearbox, that then comes under altering the monocoque of course.
Just me thinking here you understand but if the alterations are fairly subtle, say a different engine,gearbox,rear axle and the gearbox tunnel cut to accommodate the bigger gearbox then is it down to the MOT man to report you if he thinks it infringes the rules or is it just pot luck if you get stopped by the police and they spot the changes.