I have lpg on my stock 1600TP - I have been to a couple of 'standard' lpg vehicle maintenance garages and have found that they have no problem with the air-cooled system - as apart from the exhaust-heated gas feed, there is precious little difference from a water-cooled carb petrol engine. they can use the exhaust gas analysis to get to a precise gas mixture setting and can check all the electrics and gas connections as standard.
big diff from injected petrol engines and those with complex electronics of course, but checking for simple problems like electrical circuits and solenoids and gas feeds should be Ok regardless of it being an air-cooled VW...
for what it's worth, my system was installed by Gasure at Chester and I was not hugely content with it - and have modded several aspects of the installation since...notably to retain the original oil bath air filter by using a BLOS lpg mixer carb instead of the 'gas ring' injector on the carb
cheers
Rob