Hi and Welcome to TBN CooterBrown,:thumbsup:
I had a G100 from new and must say it was a great machine, everything ran well when I sold it with 300 hours or so on the engine after 5 years of great use. Mine had a hydrostatic drive, so I imagine that you could do well with you ~20 Briggs as long as you cut normal lengths of grass. I dont think mine every really bogged down even with long wet grass (I dont really have hills). The trick is to make sure that your Briggs drops right in if it does my guess is that you should be ok provided you dont really have hills or cut a lot of long wet grass.
I am not a mechanic or a gear head but I would try it, as I recall a 25 hp was about 1K. Let us know what you do and how it turns out and post some pics of the steps