Here is the illustrated parts list for your 121702, which was made on 9 August 1990:
http://www.m-and-d.com/pdfs/briggs/121700-ms6592-0998.pdfHere is the illustrated parts list for your 12F802, which was made on 3 September 2003:
http://www5.briggsandstratton.com/eu/en/pdf/illustrated_parts_list/100%5CMS0517.pdf
As you thought, they are both 12 cubic inches displacement (200 cc). The 121702 is a second design generation engine, with a vertical crankshaft, a Flo-Jet carburetor, plain main bearings, and a rewind starter. The 12F802 is a 16th design generation engine, with a vertical crankshaft and a carburetor that was not made by Briggs and Stratton. It has plain main bearings and a rewind starter.
You can look for design differences in the illustrated parts lists, but I doubt you'll find anything very definitive. Essentially, Briggs' 9 cubic inch engines with the Pulsa-Jet suction carburetor were 3.5 hp in the 1970s. With the Vacu-Jet carburetor in the 1980s they were only advertised as 3 hp. The 12 cubic inch engine would have been about 4 to 4.7 hp if it had the same output per litre. Your 121702 therefore had a particularly low advertised hp for its size, while the 12F802 was relatively high. However you should bear in mind that in between the two manufacture dates the engine manufacturers became keener to advertise the highest possible power figures. Back in the old days they used an extremely favourable way of establishing what power rating they would claim, and they may have become even more ambitious in this respect along the way, until they were successfully sued about it fairly recently.
