John, buy sams lysholm charger in the topic lysholm charger. Will make better power.
1341cc bottom end would be good but if you dont do it yourself a full rebuild will be around £1000 including wossner forged pistons. TSR sell them at a good price.
As for headwork, where you located? Themanintheshed is the man to use. He does a lot of head work for lots of companies and knows his stuff better than anyone. My head was done by him, not big valves, and a few other guys off here have used him. A ported and polished head is around the £500 mark, big valves is quite a bit extra. I never got big valves and my full engine rebuild done by TSR was just over £1500 incl vat.
Cam would be ideal as well, £250 for schrick, G60 injectors £100?, Gt inlet £40?, 51mm throttle body £100+ exhausts system/manifold any price really but in the last group buy PPP full system and manifold were £800? Re-map - £300. Might need a good clutch as well and there around £300 at a guess.
Thats over 3k and then you got to buy the charger, bracket, pulleys, extra pipework, intercooler. If you want bigger valves probably extra £150-£200
Also you would want bigger brakes for the power, and a subframe would be a good idea.
Would make some money back buy selling charger and that though.
I'd of gone turbo if mine weren't an original G40.