If your P2 casings are good , then go with a W5 Malossi 210 & W5 head - add the RZ righthand exhaust & a 28 or 30 mm PHBH - a banded 23tooth Cosa Clutch and a modified PK XL flywheel - always use the later EFL gearbox - use a Racing Mazzuchelli crank and all new bearings - use a new Genuine Piaggio cruciform and selector rod - all new German or equiv bearings - Genuine Oil Seals.
Match your casing ports to the barrel.
If you are a small guy stick with the P2 4th gear , if you are 'big boned' lad then use the late PX125 / T5 4th.
Gas shocks front and rear , and a disc brake for the front [ conversion forks etc].
All of this is available from SiP on this site , and is the lower end of the kind of tune you have described - cost it up and then decide how far you want to go ?