For what it's worth, if I had to put in a decompression valve on any big bore bike I would not waste the time and money using 13mm ones (standard spark plug size). I will be changing my Maico 490 over to a 18mm one as soon as I can. The standard size units still don't give my bike enough compression release to make starting that easy. To start any bike bore bike they need a decent kick to get the fastest rotation of the motor as possible, (fuel suction, voltage/spark generation) and a 18mm decomp valve should give you maximum pressure release in your combustion chamber. I believe that the UK Maico owners tend to use the 18mm units over the standard.
Want any Vulcan Work Auto Decompression valves give me a call.
Kevin