First step is to make sure everything is working properly. Cars that old can have a heap of sneaky little things slow them down, like dodgy earths, a partially blocked exhaust, a stuffed vacuum secondary diaphragm on the carby. Also check for vacuum leaks and make sure the distributor advance mechanisms (both centrifugal and vacuum) are working as they should. Valve clearances need to be right and even carbon buildup on the backs of valves and in the combustion chambers can cause issues. You may have already checked all of these but if not any or all of them could be knocking the edge off your L20B's performance. And that 's just off the top of my head. There will be others.....
As for spark plugs, NGK BP6ES are spot on. Even if you change cam and carbies, they're fine.
Extra static distributor advance should only be tried if you're running higher octane fuel than standard. Be careful if you experiment with this.
A better coil may produce a small gain (a couple of bhp) but they usually aren't cheap. It's not legal to change the carby or disable the anti-pollution gear, although blocking the EGR (Exhaust Gas Recirculation) should give you lower intake air temperatures. You can run a cold air intake duct to the air cleaner. A good exhaust system (with the stock exhaust manifold) can help.
Unfortunately there are no simple tricks to get 20bhp more than stock. I wish there was. All you can do is make the most of what you have.
Good luck!
Hi, thanks for the info, it is exactly what i needed to know, have checked the earthing and it is good, will go over the rest on the weekend,