The exhaust on the 25's usually has a bridge in the middle and is a different shape from the 30. Most 30's have a big open exhaust port.

The intake port is essentually the same on them.

The bolt patterns intake and exhaust are the same on both the 25 and 30.

There are some variations on them over the years. You may have a 25 & 30 side by side and the crankcase is heavier on the 30 then again you may have two and the crankcases are exactly alike.