Hi David
Well, you are half correct. There were two different models of .18tz engines. One had the pull start and normal front end that would accept props. You had to grind off the pull start nub on the end of the crank pin, then order a regular backplate from your favorite O.S. Dealer (or make one).
The second variant, looked similar to the engine in your post. They are the version that came with what's referred to as the "Serpent" crank. The front bearing portion of the shaft was 7mm if memory serves. So, you have to find a collet and drive plate from an engine that used the same crank dia., or have a set made. I found one that came off an Irvine .15 car engine. The next item, is making a sleeve nut for the crank, as the threads are much shorter on the Serpent crank engines. You also cut off the solid part of the crank, just ahead of the threads. If you have access to a metal lathe, and the skill, you could make these parts yourself. Or, perhaps someone listed in the business section of this forum would take on the project.
Good Luck
Les