Many thanks to all who have responded, and special thanks to Graham for his generous offer to cast a replacement part! I am not sure yet whether I will have to go down that road, but in any case I will take a mold from the crankcase part before I start cutting metal.
My fist attempt to fix the over-bored part will be the JB weld method proposed by Tug, although I am not 100% sure about the outcome. Did some CAD modeling of the engine:
and I think the highest stress on the crankcase walls will occur at the places of the little red circles:
While there is room for a reinforcement at the crankcase side walls, there is unfortunately very little room between the crank web and the back wall:
So the reinforcement can not go "around the corner", and I suspect the crankcase might fail at exactly this point. But we shall see...
That being said, my confidence in the Kratmo as a useful engine has dwindled over time. Biggest concern is the long and thin crankshaft, together with the large overhang from the front bearing. Any engine put on an RC model will have ground contact with the prop sooner or later, and even the slightest touch to that large prop will cause a bent crankshaft. To make the inevitable repair even worse the connecting rod is permanently attached to the crankshaft, a feature I have not seen on any other engine before.
So my current plan is to build the engine and see if it runs, and then it will probably end as a dust collector on the shelf.
My castings are almost certainly from the one of the replica batches, they just look to good for 1935. The drawing, however, seems to be a reprint of Walter Kratzschs original drawing. It is definitely hand-drawn, not produced by CAD. Print quality is rather poor, partly. It came with four pages of build instructions written by Walter Kratzsch himself. They are in German with a print quality far to bad for optical character recognition, so probably useless for most. But there is nothing special in there, and for an experienced engine builder the drawing alone should be sufficient. Got it from here:
https://manualzz.com/doc/4430645/antik-plan-liste---modellbau-and-kopierservice-gerold-kirchertThey also have plans for the smaller Kratmo engines, and a few others.
Wolfgang