I have a mountain of ex-CDN comms stuff thanks to some dumpster-diving of local Reserve units when TCCCS came in, but though I'm familiar with the MX-2799 'box base' AMU etc., I can't picture this 'brush guard' you're talking about. Have any pics? Usually they just mounted to the rear corner of either the M151A2 or M38A1 on the same 'utility bracket' you used to mount just about everything else - just without the bolt-in ring adapter you'd use with the MX-6707. IIRC the MX-2799 went bye-bye when we got the roll cages fitted to the M151A2s and double MX-6707s were fitted, but as with everything, "standard" was a word that just didn't apply.
Your KY-57 -- is that one of the demilled ones offered by American Milspec and a few others, or a more or less complete one? KY-38 units show up from time to time on Ebay with almost all their cards intact less the ones pulled during demil, and the last one went really cheap too (I was out of town or it wouldn't have!) but I know of nowhere you can buy one right now. My unit (744 Comm Regt.) still had KY-38 NESTOR stuff in stores as late as 2000. Even as a buck Pte. I knew that stuff was looooong obsolete and long overdue for a date with the C&E Museum.
If you're looking for mount brackets for the KY-38 they're are all over -- IIRC they're the same ones used for the KY-65s in GRC-106 setups and FAC racks like the GRC-206 on the U.S. side (EDIT: No, they're not the same! Just checked my own KY-65 mount and the KY-38 won't fit!), plus some outfits used them as 25/77 packset mounts on motorcycles and what have you. There's a great pic of an M151A2 in Gagetown c.1981 with the KY-38 installed on the radio box floating around on the web somewhere for reference purposes. I just went with the KY-57 setup on my own M151A2 CDN as I think Reg Force would've had them when the 151s were still in service. I know I never got to see one while I was in.