Can Briggs straton 8 HP model 190402 type 0992 -02 code 79052410 be converted to 12 volt starter or is the very expensive 120 V type the only alternative?

You just need to get a 12 volt electric starter, it interchanges with the 110 volt starter. You should be able to use a starter from any 8 hp to 12 hp single cylinder flathead Briggs engine, 1970s to 1990s vintage. If you need the starter ring gear, you can purchase it as kit with mounting screws to attach to existing flywheel.