Stick a cooper s gearbox and driveshafts on there. Get a proper lsd.
Machine out the throttle body and install a bigger butterfly.
Full exhaust, cam, cold air intake and a proper tune.
Fit a better header tank and make sure the cooling system works properly.
A multitude of coilovers are available.
Get the lightest wheels possible, 15" R81 style weigh only 5.5kg if you have standard size 280mm r50/53 brakes, fit some wider sticky rubber on them and it will be transformed. If you have bigger brakes then get some aftermarket 16" rims, as light as possible. I can't overstate how much difference saving unsprung weight makes.
Think about weight saving too - fibreglass panels (doors save a lot), get rid of all stereo, rear seats and belts, etc., half-decent bucket seat (and remove passenger altogether), race battery, etc. but do sums vs weight saving to work out what's the best bang for buck. Lightest wheels you can get away with, some 15s will go over R56s brakes for example and tyres for 15s are also cheaper (as well as overall weighing a good bit less).
Fully renovate front and rear suspension ideally so you can get the most out of the tyres, so poly the subframes and coilovers. go stiff on the springs as well so slicks can definitely get worked enough to get up to temperature.
I think a cam would help and you don't need to do head work - though does anyone know if an R53 head on an R50 with a racier cam would work? They have bigger valves don't they? Cat delete too maybe as you don't want backpressure in an R50.
I'd keep the midland box as I think it's lighter, also I think some people remove 5th gear from the 'box as you never need it on track? LSD will definitely help too but they are expensive.
Some of it will depend on the rules of your series though!