DDK - Die Deutschen Klassiker

I have a 901 gearbox from my 914 which has been standing for 25+ years.
I can select the gears and the input shaft & output drive shafts seems to rotate ok.
I am getting to the stage in my 914 restoration where I will be ready to install the engine and box.
I was wondering if there is anything I could do to prep the box?
I will obviously be changing the oil but could I/should I try to flush it with anything?
I don't really want to strip the whole thing down as previous owner assured me it was ok when car was taken off the road. He had built a new engine and done rust repairs then stalled in his build but said the box was good and was just going back in. But that was a long time ago and it has just stood in the corner ever since.
Advice appreciated.
Thanks
Jon

Not very positive, but I've seen engines red with rust inside when stood still for a long time.
The gears could be rusty as any oil film would have drained over that time unless the box had been tuned to cover everything in oil.

Maybe a call to Mike at Bainbridge Engineering?

Good points that i hadn't thought of. Perhaps I will take the end off and take a look.