I like the idea of a side rail but I'm not sure I want to alter the Maadi.  Thoughts?

If it's a pre-ban Steyr Maadi, I would say absolutely NOT! ACC/Intrac or Century may be a little less "rare," but it's ultimately up to you with respect to maintaining originality if you ever plan to resell it. Can't tell you how many I've seen dropped in polymer Tapco stocks with goofy muzzle brakes and Walmart Krylon finishes for sale on Gunbroker. These are mostly all reversible changes, but not so much after mounting a side rail. My MISR-10 is staying bone stock and also staying with me, because I enjoy the vintage AKM look and history of the Maadi...your vision and desire to keep/sell may differ.