First of all if you want a 6:1 engine you need to buy it that way in the beginning. The gearbox doesnt just go on the engine. The crankshafts of the 6:1 engines have a gear cut into them. Then the gear reduction box mounts on the side with a large gear that is driven by the small one on the crankshaft. The large gear is conneced to shaft that comes out of the gearbox and drives your aerator or whatever.
Second, I am pretty sure the gear reduction parts would not be at all similar between a Honda and a Briggs. I mean they look very similar but as far as being interchangeable at all I highly doubt it.
But it sounds like you bought a normal engine and expect to put a 6:1 reduction on it. That is not going to happen period...unless you change the crankshaft and buy all the 6:1 parts. For a honda that might include changing the side cover.