If you can't get the stock engine running, the "cheapest and fastest" alternative would be to find a running engine from a 240 or 260Z and swap it in. Hopefully you've figured out that the L24, L26 and L28 engines are nearly identical (except for bore and stroke) and can be swapped fairly easily between cars - much easier than swapping in a V8 or RB motor.
The 240Z engines (aka L24) are fairly easy to find because everyone with a 240/260Z wants to swap in the bigger 280Z motor, and unlike the L28, the L24 is a carburetted engine so you don't have to mess with swapping in wiring harness and electronics for the fuel-injected L28. Look in the classified section on this page - or check ebay, craigslist, local-junkyard, etc. I've seen people here offering to give away complete L24s just to get them out of their shop...
This website
http://datsunzgarage.com/rebuild/ will give you an idea of whats involved,
in removing/replacing an engine, but get a Factory Shop Manual (FSM) for detailed directions. Its not all that hard to remove/replace an engine, but its easy to hurt yourself in the process if you don't know what you're doing.
Don