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.
Cheers,