Hi Dave, Welcome to 3 Wheeler World!
There are a few sites that sell aftermarket exhausts for the 200 Big Red; my favorite site for aftermarket parts is www.denniskirk.com.
As for fixing the high low sub tranny it is hard to say what needs to be fixed until getting in there. There are not too many parts so I doubt it will too expensive. As for the easiest... that is easy... run it in high only (I always run mine in high, only would need low for pulling heavy loads).
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110