Yeah ive seen a few aftermarket swing arms for 79/80 RM models (mainly only in literature) but they are REALLY rare. You wont find one, not many were made/sold as they had good alloy swingarms standard. They almost only exist on paper if you know what i mean. if you want a trick swingarm GMC can make you one and annodize it gold.
How much of a bike have you got? if you just want to get it going and looking 'about right' there is so many options for suzuki forks to use. There is always heaps of forks and triples at wreckers and on ebay and not too expensive either. Pretty much any 78 to mid 80's leading axle RM/DR/PE 36 or 38mm front ends will go on with not too many dramas.
First thing i would do is check out the gear box and rebuild the motor before you get stuck into the rest of it. RM gears are fast becomming extinct and hot property, there are guys struggling to find them right now, i think its first and second gears that wear/break?