Railhead Publications has begun selling a Sn3/Sn2 Center Cab Diesel Body. The body is designed to fit an older Athern, Atlas, Kato, Stewart diesel (on Sn2_Trains yaho group we have discussed converting Kato NW-2 locos to Sn2/HOn3). The body fits onto frame of the doner mechanism.
The kit is laser cut by Mount Blue. Actually, it is a scaled down version of the Mount Blue O/On3/On2 kit.
Current cost is $34.95 and is available from Jim Eakin of Railhead publications (P.O. Box 6579; Canton OH 44706) (to my knowledge, Jim is not on the web, so just send him a check)
This kit looks like an easy way to get started in Sn2.
Update (Jan 15, 2011): David Keith is in the process of assembly one of these kits. The body will not easily fit on the Kata NW-2 mechansim. It would require significant sanding of the weights to fit. As such, Dave has ordered an older blue box Athearn SW1500 kit, and hopes to use Jim Eakin’s techniques of MR Oct 1980 to regauge the mechansim. Will update when it arrvies.