I found this bike on a friend's shop; a Maruishi RX-5 aparently form 1990 since the serial ends in 90; double butted Tange 1000 tubing. It does seem quite nice, and I can probably get it very cheap, the equivalent of $100USD, but my main interest on it is it's size, it's damn hard to find a large bike in this country. So, I see there is some rust but i'm not sure if it's just cosmetic or if this really is an unrideable bike

I took a couple of pictures hoping your more experienced eyes could give me a clue. Some rust remover and linseed oil will do the trick? or should I steer clear from this piece of crap?

Now; vintage and rust porn: