GMC make a VF414 pipe which is the same as a normal VG414 pipe. It should be reasonably
straight forward for someone skilled at 2T pipes to reroute it. Looking at other 414 into VG250 frames they appears to have been done by altering a standard pipe. GMC also make an alloy silencer but I don't think that would go anything like fitting although a 490 maico silencer would be close I would think.
As you know the Monty's are very sensitive to pipe design. The GMC pipe from the only report have been told it is outstanding quality but does alter the power delivery a bit. It seem to take some of the bottom and gives more mid range and top end. The 414 do not lack mid range to start with so how much the altered curve affects ridability I am not sure.