Yes I am a crusty sod, the crust comes of years in the bike trade followed by the despair at the things some folk waste time and money on.
each to his own or à chacun son goût as the frogs say.
I am 90% that your engine is an Ydral; these engines were quit ewidely used in all sorts of vehicles from invalid trikes, to mini cars or velo moteurs in which France abounded after the war. That's the 1939-45 one not the Falklands which is about as far back as some's memory goes.
They seem to have been reliable enough units so engine stuff is possibly easier to get which is half the battle.
If you spoke/read french you could search ebay.fr, don't bother with ebay uk, for issues of Moto Revue or RTM revue Moto Technique which over the years covered such engines in depth, as near a manual as you'll get.
There are three dealers over there who supply spares for all sorts, their names are on here; They are OK to deal with, accept cards and paypal. Postage is a killer if its a parcel.
I have bought lots from ebay.fr when the exchange rate was in our favour; never had a problem with seller or post, just check they'll send to here and take paypal first, 'cos lots don't.
Start all enquires with "SVP Monsieur" as it helps a lot.
As was said they aren't keen on exporting their heritage, I have seen so much old shit sold to us rosbifs.
Over the years I ahve owned
1930s Terrot LST
1947 FN 450 with the rubber suspension.
A 250 Peugeot
a Motobecane Z22c 175cc which was a far better made bike than a tiger cub
a Motobecane Z2 125cc
a Manhurin scooter,
and numerous mopeds.
The mopeds sell readily enough to mugs here, all the others got sold/traded back to France, the best place for them.
At least yours goes. I hope you got it for nothing.
Just about all your likely problems and their answers will have arisen in the Terrot saga. You could share his patience.
That's my lot, if it offends, I don't care that's life.
I think there should be a separate section for french bikes because of my blood pressure.
1920s French flat tankers are a whole different thing.
I have a massive knee op soon, so I think I'll dig out some of my pics of interesting french bikes and scan and post them, maybe some of the crap too.
I still got a pile of french bike mags that are destined for the binman soon.