Are they B/S flattys? If so, check the model #s. The 60000 series motors had a shorter stroke than the 80000 series motors. I'm not sure if B/S made up the difference by either a taller deck height or a shorter con-rod length. Maybe it might be best to rob the goods off of the 2,5 to resurrect the 3 hp, if possible. I've ran the 60000 series and 80000 series motors in place of each other on different creations, and all other things being equal, the difference is very noticeable.
If you ever come across one of the B/S 3.5hp flattys, snatch it up. They have 9cu. in. displacement (vs 6 or 8) and are almost the exact same outside dimensions.
