I'm with the previous poster -- consider getting a "real" karting engine. They're not necessarily much more expensive, if you buy used. And you're much more likely to find help at the track (and karting shops) when (not if) you have engine problems.
How old is your son? For example Raket 85 engines are not much more expensive than that, and you can get it with an engine mount that bolts directly onto the frame. Sure, they're direct drive, but I'm not sure if the Motonet engine has a clutch either?
An easy option for a younger kid would be the Raket 60 "Cadet" engine. It has a clutch and is quite easy to operate, and maintains its resale value really well. If you buy a used one, you will probably get your money back when selling it.
Try posting a WTB-ad in the "ostetaan" section of the forum.
Edit: Check out this ad, he is selling Raket85 engines for 50eur/piece
http://www.kartingforum.fi/forum/index.php/topic,14619.msg52704.html#msg52704