I think the best person to talk to is a very nice guy called Olivier, he races bikes in France and is after an engine and some replica exhaust the same as in the photo. You can visit
http://www.petochon.fr/menu.php and speak to him there, he is very enthusiastic about racing one of these, and probably has some knowledge to. I have tried for a year to get him a motor but to no avail, could have got him a TT super sports but he wants a Brooklands. I think the chassis is a 350cc Monet Goyon. I don't think is,t M Barthe, that's me getting it wrong in translation I think he says it,s MG Albert Sourdot.
Please contact him as he is a really nice helpful guy.
All the best
Mick