I have the original headlights on my 74 Superbug which are working but are nowhere near bright enough.

What's the best way to upgrade my lights? H4 conversion kit?

You can buy H4 globes to suit your head lights, make certain all the earth contacts are clean as the get corroded over the years.
http://www.classicandvintagebulbs.com/page3.html  P45T base, these are avilable from this site.

Check the voltage at the headlights first, the best headlights in the world are still gonna suck if the voltage is low.

Best upgrade is just replace the original non sealed headlights with some H4 semi sealeds.
They bolt straight in if you get the ones for Beetle,Kombi/Mk1 Golf.

Cheers for that.

A mate of mine has some Mk1 Golf headlights I can try in there.

I'll check the voltage while I have it all apart too.

Just fitted the MK1 Golf ones to mine. Huuuge difference!!
Better range of H4 globes available too.

Yes luckily most replacement ones have the common P43t base, but all the original German ones were P45t.