Ta will have a look at that and for serial number,its quite an old machine but worth a look . Cheers 
My guess is you have either an 800 or 900 series B&S (3.5hp) engine if it's driving a rotavator that's a few years old.
This bit of info may help:
Briggs and Stratton engines are identified with a MODEL, TYPE and CODE number system and take the form of the example below.
Model Type Code
12F802 0637 01 99070759 The MODEL number is either 5 or 6 digits which may contain letters
• The TYPE number is 4 digits followed by an extension of 2 digits. Letters may be used in the extension
• The CODE number is the machine's serial number. The first 2 digits are the year of manufacture
(the example above was made in 1999)
The numbers can be in a variety of locations depending upon the engine but are generally to be found either:-
• Stamped into the front, top or side of the main cowling
• Stamped into the plate over the exhaust
• Etched into the plastic cover
• Stamped into the rocker cover on an OHV engine
• On a riveted plate on the main cowling
Sometimes, plastic covers or excess paint may obscure the details but they will be there.
The only exception is if the main cowling or part holding the details has been previously replaced. In which case the details should have been retained from the old part.
I don't think I can help more than that without actually seeing the engine or knowing what rotavator it's bolted on.
Good luck!
G.
