I have a MR1 and I love it. Benelli makes absolutely amazing products and their ARGO piston system is superb. Benelli pwns shotguns, and no doubt they tried to apply that same mojo to rifles. However....marketing, marketing, timing, and more marketing.
The RX4 and MR1 are the exact same rifle with the exception of the buttstock and barrel.
You MIGHT be able to find the collapsable stock on gunbroker, I've seen them there. It's basically the same stock off the Nova shotgun, but I have no idea if there's any difference that would prevent installation.
The forearm rails are available from Benelli and you might see it on either Brownells or Midway USA.
The problem you'll have is the barrel. You'd have to figure out a way to get it from Beretta, somehow. You can't just put a flash hider on there, because you wouldn't be able to remove the retaining nut to disassemble the rifle. Then there's the thickness...repeated rapid fire probably won't melt the foregrip--but it'll sure overheat that pencil thin barrel in a hurry. The RX4's barrel is a completely different contour than the MR1.
Also I don't know of too many US parts/accessories manufacturers for MR1 aftermarket stuff (like zero). So 922r compliance would be difficult. Most of the RX4's you see in the states are just that--RX4s. If there's a LEO conversion for the MR1, my recommendation would be to buy it like that outright.