Hi Albert, welcome to Outdoorking.
A 3.5 hp Briggs could be either side valve or overhead valve. Where the numbers are located depends on what kind of product it is used on. Here is the B&S instruction for finding the numbers on a push-mower (that is, vertical) engine:
http://www.briggsandstratton.com/us/en/support/need-help/push-mower-enginesHere is the equivalent sheet for a snow blower (horizontal) engine:
http://www.briggsandstratton.com/us/en/support/need-help/snow-blower-enginesHere is the sheet for a general purpose small horizontal engine:
http://www.briggsandstratton.com/us/en/support/need-help/other-enginesBecause your engine is only 3.5 hp, we don't need to worry about the locations of numbers on the larger engines.
If you have any problems finding your numbers from those instruction sheets, please tell us and we can find another way to get the information - we would probably need pictures of the engine as a starting point.
When we have the model, type and code we should be able to direct you to the Operator's Manual and Illustrated Parts List. If you wish we can also refer you to an appropriate B&S shop manual.