Depends on what you have access to - I'm a fitter so just about anything is possible (well... how much time do I want to spend on it is the question here) but I'm not sure what you have access to.

This is what makes the cost vary heaps - how much work you want to do/can do your self.

Common NA mods involve...

Cams - more duration, lumpier profile.
Porting - clean up and open up bowl/guide area, port matching
Heads - find your self a A14/A15 oval port if you dont already have one
Exhaust - headers (or even stock manifold) with a 2" system
Intake - goes hand in hand with carbs
Carbs - Webbers, SU's or to be different; motor bike carbs (looks best!)
Pistons - Hi comp (forged if you have money to burn)
Crank - Polish and knife edge
Rods - Polish and lighten
Flywheel - Lighten plus drill
Springs - higher shut pressure on the valves
Retainers - Ali or titanium valve retainers
Push rods - lighter ones
Crank Pulley - lighten, smaller for less fan rpms
Rebuild - rings, seals, bearings, oil pump, water pump, ARP bolts
Roller rockers - only really good if you have cut valve guides or tons of money for bling and requires the mandatory perspex rocker cover!
Sump - baffles + larger
Balance Engine - blue print rods too
Bigger Radiator
Electric Dissy

Last but not least a TUNE! Get the carbs running smick and set your timing (to what fuel you want to use) or all that work is wasted.

I think Ive covered all the mods beside silly things like making custom cylinder heads or full race mods like larger valves, etc.