Casey: I would take both those FN-49s. clean them right up and put them back in the closet or safe. I wouldn't take them out hunting. They are as much a 'klunker' in the woods as are my AG42Bs (Swedish semi-autos). Both are still WWII style semi-auto rifles and pretty long and heavy to lug around in the woods of upstate ny.
As to your magazine questions - in the old days we just used to make up wood plugs to fit in the bottom of the magazine to reduce the number of rounds it could carry down to the legal limit. If the FN magazines require some permanent modification I would most strongly suggest you not do that. I do not know of any factory made 5 round mags for that rifle. Be that as it may, as long as the magazine is limited to 5 rounds you should be as legal with it as if you had taken a SMLE 303 Brit with a 5 round magazine/magazine plug out hunting.
The FN-49 is not listed as a assault rifle on any list that I know of but, it is a semi-automatic weapon with a bayonet attachment, which would probably make it as illegal as any anti-gun fool in nyc could imagine.
Let's also look at the rifle itself and the ammo available for it. This is a older action/rifle and I do not know how different mil-surp or current factory ammo with treat it. I would not take it out until I installed new springs (esp recoil springs) and had the headspace checked. This rifle may not handle the wuzzy American loadings for the 8mm (7.92x57mm) and may not cycle the action with those loads. Some of the European factory loadings, the Sellier and Bellot, PRVI and mil-surp may even be too hot for it with their 196 gn loads at 2500'/sec in those older semi-autos.
I like the 8mm and will use it for anything on the NA continent except big bear. If you are thinking of a 8mm, there are lots of mil-surp bolt guns that would probably be less expensive to sporterize than to purchase one of the FNs. If you are thinking of a semi-auto I think you would have to look to the European market and deeply into your wallet in the process.
I think, just fer the hay of it, that if I ever found myself some shot-out old 06 in semi-auto I would probably have it rebarrelled to 8mm, but finding a solid enough semi-auto sporter in 06 is a chore. HTH.