The same boss that fits the Briggs will fit the Honda - provided you get the right version of the Honda engine. The same basic engine is fitted to pumps, generators, pressure washers, etc, as well as mowers. As long as you get the version with the right length/type/diameter of crank shaft you will be fine.
Mountfield fit/have fitted Honda engines to some of their more expensive mowers.
phil_saunders(a)bigfoot.com
Phil Saunders
September 2010