Helge, it depends on what you want to fly...
Of course the engine will be OK for flying if you have a lightly loaded old free-flight design flying as RC assist.
Almost all my flying is like that, and I mostly use old engines that are not running at max. performance.
Just avoid heavier, faster flying planes that need more power and you won't have any problems.
I never worry (or even think) about "max. rpm" with any of my planes, just use a good reliable old engine, that's well matched to the plane,and it will fly OK!
At the moment, I'm flying at least three old dieselsthat are older than your DA.