They would have determined that this is the direct replacement from the Briggs dealer/distributor site. Both 31C models are 17.5 hp, the difference being the back up manual starter on type 3005. You could upgrade to the 331877-3036 for a few dollars more and get 21 hp plus pressure lube and external oil filter. The wiring hookup and crank shaft dimensions are the same.

These engines are all physically the same size. The alternator MAY be different however, they will interchange although I am not sure you want to messing with an engine that is in warranty. Crankshaft and mounting holes are the same. You can go here and check out the internals of your old engine and any prospective engine. http://www.outdoordistributors.com/Briggs_And_Stratton/briggs_part_lists.html

Walt Conner

By the way, none of the engines listed including the 312777 are Vanguards not that it matters.

Walt Conner