ZM 2000,
I believe you are right about the cooling method on the sprayer, you would expect the hot air to be directed away from the operator. so there may be a fan on the output end of the shaft.

However, all this still does not address cometravi's problems, and I think that unless you are an engineer with your own equipment, or have a friend who can do the job, it all could be rather expensive and needs to be balanced against importing a weedeater motor, (maybe secondhand? ).
If you persevere, I'm sure you will find an answer, your enthusiasm should carry you through.

Volksman,
50cc motorcycles would make great aircraft engines except that the cranks are normally full circle types which are extremely heavy, and of course you might have to change to a pumper carb as you say.