I have not found any glue that will perfectly repair the plastic fuel tanks. The best that I have found so far is 3M Scotch-Grip 4475 Plastic Adhesive, but I am not 100% satisified with the results.
All of the plastic fuel tanks are interchangeable with each other i.e. a plastic tank from a 5000 will fit a 4600 and a 3800.
I have had good success using the black tanks and rotor hoods, usually available from www.velocruz.com, and painting them with Krylon Plastic Fusion spray paint, gloss white. As long as you follow the directions exactly, they come out pretty good. Look at the Solex on my profile page. That white tank was a black tank painted white.
Also, generally the 3800 didn't have a white tank. Are you sure you have a 3800? Any photos? Motor, frame serial numbers? 4600s and 5000s also used the Cyclomoteur 3800 motor if that's what you're going by.