I've got the same question myself but so far it's looking like "yes, but not without access to some machining tools" because you would need a way to extend the shaft of the motor a bit to accept a clutch. I'm working on just such a thing for my yardman ym400. I've pulled the center shaft off and I'm trying to find stock that will work to build a new center shaft that will extend out another inch and 1/2 then add a clutch from a chainsaw.
Not sure how long it will take but let me konw if you come across anything.
I'm also using a yardman weed eater motor. It's 31cc. I was weed eating earlier with a weed eater brand weed eater and noticed when it was idling it was still spinning. Is this going to cause a problem if my yardman motor does this on my friction drive set-up?
You could add a compression release valve, allowing you to turn the engine over without compression for just plain cycle-ing, and firing up the engine when needed. I think thats what wartime whizzers did.
I've found that indeed the YardMan ym400 and similar small and cheap motors do not have a clutch on them. Also, be aware that finding parts for the cheapies is not so easy. I tried getting piston rings for mine and they only want to sell the complete piston with rings on to the tune of about $45 shipped. Better off buying another motor.
I'm now looking at getting a similar sized but higher end motor liek a Ryobi or other with clutch already on it. These can be found on craigslist and eBay for around $40 and availability of parts is much higher.