I think you are incorrect about the .91
How did you determine this?
You probably have a Sportster .65 which has a 91 stamped in the back cover for the year the casting was made.
Sportster engine family consists of 3 sub families. The .20 & .28, the .45, and the .65
Distinguishing features: 4 bolt head, Made in USA on one side of case, K&B (not oval logo) on other side. Some had size stamped on mounting lug but most had no size reference. Plain bearing engine. You can determine the size by counting the cylinder fins... the .20 & .28 have 9 fins, the .45 has 10 fins, and the .65 has 13
more info here
http://www.mecoa.com/kb/parts.htm