Well, lets double check the manual since I don't happen to have an MR5 in front of me....
They of course generally don't mention aftermarket compatibility, so lets just check the parts diagram.
Ok, good news and bad news. Bottom line has standard in-line holes, not the old spyder offset holes. So any ASA that attaches to an in-line hole pattern, or any rail mounted ASA paired with a add on rail, should bolt right up. I have a thing for CP ASA's myself, but whatever your preference should be fine.
The issue is the hose. They still like to use oddball hose threadings. So you'll need a new hose as well. On the gun the stock setup has the ASA, a male-male metric adapter, then the hose, then to a male-female metric to standard adaptor, which screws into the gun. Ditch the hose and male-male adaptor (stick them in your parts collection, never know when they might come in handy), leave the male-female adaptor screwed into the body, and screw a standard threaded paintball gun hose into it. Stainless braided is stronger, but you'll have to figure out the right length. Macro-line isn't as strong (I generally don't recommend it for CO2 use, since it's output pressure goes way up when it's warm, but fine for HPA use), but can be trimmed to the exact desired length.