Mike,
You should take a look at both engines before you buy. Maybe even take notes and photos. The 250C has the single cam M130 six and the 280C has the twin cam M110 six. I don't think any engine parts are interchangeable. As to swappng engines, you need to see how much of the accessories, motor mounts, wiring, plumbing, exhaust system, etc, you will need to convert to fit the later engine. Can you run that 280C engine before you buy? Make sure it's reasonably quiet and has good hot oil pressure.

Happy Motoring,
Mark

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DrDKW