Have a look here: http://www.miyatacatalogs.com/
You'll see the elevation 200 on the main page in the 1994 catalog. Seems like a good bike with chromoly main triangle and probably a medium steel rear end... chromoly fork. I don't see any reason not to convert it. It's not the lightest MTB frame out there but Miyatas are very well made and if you are building an extracycle there is no need for a lightweight frame, I say go for it.
1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear