Are you sure its a 'v' twin? I would think in '87 Briggs was still building the old opposed twin. It could be a replacement though or the mower is not quite as old as you think it is.
A Picture of the engine would help. Are there any decals on the mower that would indicate horsepower?
If it is a V-twin, Jeffcat is right, most had the numbers on the top of the valve cover. If one of the valve covers says AHO instead of OHV, the numbers are underneath.