I have a similar Diana Model 24, made by Diana in Rastatt
, Germany. Diana rifles are sold under the RWS brand in the US. It's a nice little kid's .177 pellet gun. My post WW-II rifle needed a crack in the wrist repaired, plus a new spring (someone had left it cocked too long), a new leather piston seal and some lubrication. It shoots just fine.
If you need parts, try www.pyramydair.com
who are Diana/RWS distributors. If the rifle won't fire, or fires only weakly/erratically, you might need to dribble some Crossman pellgun oil into the air port, and let it sit overnight so that the leather piston seal soaks up oil and expands to fill the cylinder properly. Old airgun seals are often simply dried out. The oil will also lubricate the innards for you a bit.
Have a mooch around www.pyramydair.com/blog
for tips and tricks on caring for old air-rifles, search for Diana 25, and you can also post a question in comments there and expect a knowledgable reply.