Realistically speaking you can get about 100bhp(ish) from the engine using a cam change, exhaust and induction mods.
The cylinder head is the key to the performance though - a properly gas-flowed head with bigger inlet valves coupled with the other parts would give closer to 130bhp at the wheels (about 150bhp at the flywheel) but the original carburettor is completely inadequate for that. You'd probably need to look at a serious overbore and longer stroke to achieve that too.
Without forced induction the biggest (proven) power figure for tuning was approximately 190bhp at the flywheel by Dallara but that had just about everything done to it, including a custom 16v head and fuel injection. It would have been totally undriveable on the road too.
Best bet for non-serious modifications would be a proper exhaust manifold, electronic ignition and a re-jet of the carburettor. Getting hold of a decent camshaft could prove pretty entertaining as the cam is already pretty aggressive.