Correct me if i'm wrong, but isn't the 98' the first year with the new 600 twin??? My guess is that you could drop in that 800 twin, but you'll have to use the torque plate from the 800 also. Basic clutch alignment is the only other thing i can think of.
If that doesn't work than a 98' 700 twin would more than likely drop right in. I'm not positive on all this, but it's an educated guess for me.