I have talk to tunermotorsports (or email, i forget) they have built a NA motor and got close to 300hp. BIG$$$$$$$$$$ Dont let them scare you with "there is no other way". The machining a lone would be big money.
This is what i would do if I nothing else to do with my money.
1 bigger valves
2 ported head
3 exhaust
4 ITBs (individual throttle bodies)
5 ECU reprogram or a standalone
6 M47 crank 2.1 L
7 pistons high compression
8 rods
9 injectors
10 camshafts
11 valve springs and retainers
anything else?