A low budget build for an L18 would be

1. Fit 44mm inlet valves X 280ZX to a V912 black rocker S2 bluebird head. This will cost approx $600 - $800 plus head ($100? from U-Pull wreckers). This is where you will get your main power gain.

2. Freshen up an L18 botttom end (rings/bearings $300) or bore L18 to 87mm & fit VG30 flat tops to the bottom end. Have valve reliefs flycut. Cost about $1200 inc cheap pistons if you do lots of prep work and shop for prices.

3. Fit a billet wade 76 degree cam and degree wheel $270

4 Fit some 40 mm webers with chokes turned out to 37mm. ($550 - $600)Plus a slab to a mate?.

This should give about 150hp at the flywheel (75 Rear wheel kw's)

After these basic mods costs climb quickly.

An alternative to the L18 rebuild bottom ned is to get a good high comp L20B bottom end instead of the L18 one ($100 at a U-Pull). There is a 1600/ L20B (Pooley brothers "Miser Racing" ) here in Vic with 90 RW Kw's. It just has a good head on pick a part $100 bottom ends.

Good luck.