Were the voltage reading done at engine idle speed ?
If so, you need to rev it a bit to get a proper reading from the dynamo, they can sometimes show nothing when at idle speed
Also, the voltage with the car off can show 12 volts but not have enough current to be able to run stuff.
Best way to check would be with revs at around 2,000 It should show around 13.4V also try with your lights on & see if that makes a difference to the voltage when the engine is running. Dynamo could be working but not quite enough.
Alternator conversion isn't a difficult job to do but it all depends on how good you are at getting your hands dirty