Easiest would be to start off with a VFC SPR as a base. After that mod externals to your hearts content.
Airsoft Gear has one SPR in stock last time I was physically in their store. But it's the crane stock version so if you want to be true to the SAM-R/SDM-R you'll have to find somewhere that has a standard fixed A1/A2 buttstock. Also you're going to have to switch out the flash hider for a standard birdcage instead of the SPR style "OPS" suppressor.
Next is a fixed triangle post front sight, a bipod, and an ACOG and you're good to go.
Also a personal thing, I like the look of "VN" mags (low pro and high speed looking and look like they belong on a DMR) but it seems like all pics of the SAM-R or SDM-R I can find use standard 30 round STANAG mags.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE