Its an odd engine to value, sibbers love 2 strokes, and the Yam 25 is one of the best. But there is a problem, with the exception of a few newish Futura's and possibly the odd questionable Seapro they all need short shafts. It will be suitable for a couple of the smaller ribs, but its a small market.
In really good condition, with a short shaft it would be worth around £1000, but as a long shaft I would guess at sub £500 and the market is limited.
To make your situation even worse, the engine you are looking for is popular, with good ones going for well over £1000 and they sell really quickly (I sold one 12hrs after it was advertised at the weekend). So I doubt a swap will work.
Put it on ebay for 99p with no reserve, good accurate and honest description and it will sell for what its worth.