I've been running a Mach 427 (silver, with a closed backplate) for a long time, and just put in a LRP ZR.30X.. MUCH better.. And I love my Machs (also in Savages, RC8T, SC8, etc..) But IMO the DM-1 needs more balls to push the gearing.. A .28 like the Losi 454, LRP Spec 3, XTM 457 Pro etc.. would all be ideal I think..(They're all the same engine btw too - or at least very, VERY close..) The Losi 454 is cheapest on ebay generally..
I only use a starter box, so I don't know about mounting. Usually engines with pullstarts have big bulky backplates that complicate mounting..
-Team Associated RC8.2, RC8T CE, & SC8 with 1.) Argus A52/0801, 2.) RB Fire 11/2058 or 053, 3.) RB Blast V9/2045, 4.) Nosram RS.30RR/JP-4, & 5.) modded Nosram RS.21RR/086
-LST2 #1 with Losi 454 & LST2 #2 with Dynamite Big Red .28
-Ofna DM-1 with silver Mach 427/2047
-TT MT4 G3