I'm no expert, but early Albin 25's had a 20-hp diesel that was made specifically for (by?) Albin. I think after that they went to mostly Volvos. Since the Volvos were green, as I remember it, I'm thinking the Albin 20hp might have been blue?
Is your boat from the early 70's? Early ones would have the fairly rounded chine aft; whereas later ones (mid-70's?) would have that same area slightly squared off (harder chines). They did that when they started putting in the bigger engines (for definitions of bigger that include numbers like 36hp and up).