I was looking at supercharging my 12a until i saw how much money would go in for relatively little power gains. The M90 was the one i sized up based on the volume. Maybe it will do for the 13b as well. Looked into the dimensions of the thing and had to get one with a small snout as the thing would have been longer than my engine.
Get a book called 'Supercharged!' by corky bell off amazon, about $40 delievered when i got it. Real good read,.
Don't know what happened to the site but rotahavik.co.nz had a guy who had supercharged his 20b, he was pretty on to it with this type of thing