Chock the wheels preferably on a flat surface, jack one rear wheel off the ground (easiest is the wheel opposite the side of the valves you want to adjust so you can see the valves rocking), put it in fourth gear release the hand brake turn the wheel to turn the engine.
1982 Camper 1970 1500 Beetle Various Skoda's, Ariel Arrow
Or take off the basket over the fly-wheel. Then turn the nut on the end of the crank with a socket and an extension bar through the number plate flap. I think the socket is 14mm?
2ltr Aircooled CU with twin Solex's & originally a 009 dizzie, but now back to standard.
I have turned engines by sticking screwdriver through fan between alt and belt pulley and turning whilst pressing firmly on belt to keep it deep into pulley for "bite" I have done this even without removing plugs but can't remember if its accessible on CU as I sold mine, but I'm fairly sure I did it on that particular engine.
The best way is to make a tool (especially if you have an auto) similar to the VW tool out of a socket with a 38mm outside diameter that then fits into the fan hub and a bolt head too engage with the slot in the hub, you can then turn the engine by just inserting a socket extension and ratchet.
1982 Camper 1970 1500 Beetle Various Skoda's, Ariel Arrow