I would be looking at the clutch possibly sticking and not releasing, may be full of crud in there, if its on its stand and you apply the rear brake does it kill the engine, if so could be the clutch.
sounds like its idling fine but clutch still engaged, its not a hard job to remove the clutch but it HAS to go back correctly, you need a guide as it could be assembled incorrectly at the moment.
look here and review cheetachromes guide for the clutch
also the wiki is a good page to get your head around how the various parts work. happy reading!!
here is a very good .pdf Peugeot user/workshop manual I use lots
1979 Peugeot 103 stock