how far are you willing to go? how much is your budget? and are you wanting to do it yourself or get a mechanic to do it?
If i had one to turn in into a revy beast, get a performance grind cam, some new valve springs, port the head, maybe a webber carbie or something decent and tunable, extractors and a nice exhaust.
Extractors - $50 from wreckers
2.25 exhaust - $250
Gaskets - $80
Valve springs - $80
Head port - Free yourself or at a guess $250-300
Cam/Lifters - $200
Oil/plugs/leads - $60
Carby - $100
all up ~$400-700...
or you could buy a bolt in engine, but if you play your cards right could be a lot cheaper.