match the intake to the cylinder as best you can just don't pull the larger hole u make completely into the cylinder cuz then u r fucking with port timing, basically round the edge so there isn't a wall blocking air.
as for boring out gurtners, 13.5 sounds hardly worth it since u need to drill or use an adapted lawnmower jet to make it work right.
to be honest the 12 mm gurtner and 14mm intake is an excellent combination just leave it if u like stock feel.
i just upgraded my stock moby to a dellorto from the gurtner and its a bit better, the bike tops out over 35 now when the top used to be about 34 with an open gurtner and a mesh screen over it for some restriction. the dellorto is a 16 with a uni filter and a 64 jet a little rich,
porting comming soon, with guide, the goal will be how do u get an usa av7 moby to 40mph without a pipe just porting, jetting and a dellorto, and super low end still.