If you keep the inboard operating, even if you need to do a rebuild, you are likely to get any incremental cost back when you sell the boat. For whatver reason, inboards are more desireable and bring a premium over the same boat with an outboard. A C27 with both an an outboard and an inboard would be a bastard that many buyers will shy away from. I would personally think the expedient of adding an outboard would end up being more expensive than maintaining what you have, and a less satisfactory solution.
A second thought is that putting a heavy outboard on the stern is likely to cause the boat to float stern low, resulting in a permanent beard of bottom growth unless you raise the waterline. You could reduce this problem by removing the inboard and tank, but a 25 hp outboard is way too big. I,ve never seen anything bigger than 10HP hanging off a sailboat stern. Seems to me there was a tread on this subject in the last two months, you might search for it.