If the head is off you are already 1/2 the way toward swappng the cam. A wild and crazy cam won't do a whole lot for an otherwise stock engine, but one of the milder performance cams (IPD Street perf cam, or others, or even jsut a stock 'D' grind cam) will perk up an engine with a 'C' cam nicely. Put new timing gears and new lifters in with the new cam - unless you like the process of putting cams in so much you want to repeat often...

SU carbs will be fine and feed the engine through quite a few performance upgrades - although at some points you might need to replace the needles. Those double barrel downdraft Weber carb conversions are not performance items - they are an adequate replacement for worn SU's on a stock engine. If you plan on modifying the engine for more power you'll hit the limit on that Weber much faster than with twin SU's. Dual DCOE Weber's make more power - but are usually overkill on a stock engine (unless they are choked way down - in wich case you might as well have the cheaper SU's).

When rebuilding a B18 engine you can get the cylinders bored to 89 mm and use stock B20 pistons - as well as sourcing a B20 head to put on top. At that point you will have spent a decent amount of money to duplicate a B20 engine - not really worth it with B20's not being either too hard to find or very expensive. Just find a junkyard B20 to rebuild instead. On a B20 you can (if you want) overbore to 92 MM and use B21 sized pistons for an extra 200 cc and noticabley more torque. Follow up the increased displacement with breathing improvements (cam, head work, exhaust) and you'll even make that increased torque high up in the RPM range.

Ignition systems are often glossed over - some figure if it's firing at all then its doing it's job. But sparking the mixture isn't quite graded on a pass/fail system. There are qualitative differences. Getting a powerful aftermarket coil such as a Mallory Voltmaster can make a difference in power at full throttle. As can replacing the points with much steadier and accurate pointless ignition systems like the Crane XR700.

Of course, all the above things are incremental improvements - you could just bypass them all and go straight for the jugular with John Parker's supercharger system (http://www.v-performance.com/). Pricey and you might wait a while - but there's not any other likely way you are going to see 200+ HP out of a B20.