With your bits cleaned and lubed, replace the piston, mainspring and guide/endplug. Put weight on the endplug and you should be able to locate the cylinder end pin. Replace the cocking arm in the cylinder slot. I did not try to remove the cocking arm or linkage because of the rivets. You should be able to replace the breech seals with a small screwdriver or similar. I fitted them with the flat edges to the cylinder and barrel and the stepped edges to face the loading tap. Replace the tap and locate and tighten up the pivot screw.
Refit the grips and then the rearsight. Gently push the rearsight down against the 2 springs until you can locate the elevation adjusting screw.
I did not really go into the trigger on this other than to sort out a bb and spring for the safety catch. There are 3 self tapping screws on the RHS of the grips. If you undo these, the grips should ease apart. The safety slides across to block the trigger. I improvised a cut down spring from a lighter and a bb to locate the safety.
I tried the gun afterwards. It was firing a little high for me at 7 yards, with the rearsight on the lowest setting. It was accurate but not very much fun to shoot. The pistol is light and has a pretty good sight picture. The grips are not very solid. The tap is a little fiddly to use but you will probably get used to it.
While this was an easy strip and there is nothing wrong with the gun as such, it is not my cup of tea. Between this and the Diana Modell 5 I did previously, I know which one my money would be on to last ANOTHER 70 years