Check out the Briggs and Stratton website (sorry, can't remember the address).
There is a section thaat tells you how to read your engine number and what it all means. That also tells you which manual you need and most of the manuals are available on-line.
We've just got a non-working 1978 Norlett Rotovator that has a 3hp Briggs and Stratton engine. Details from B&S have been easy peasy to get. On the other hand, trying to get anything out of Norlett is nigh on impossible (unless you are fluent in Norwegian).
As an aside, does anybody have a user manual for a Norlett Extra 5000 rotovator?