HI,
If the Iona you want was built by Thomson, Glasgow, you will find the plans for her in Rankine"s Shipbuilding Theoretical and Practical (1866) vol 2 the plastes volume--Plate E/1 and E/2 elevation and sheer, body, half breadth, deck measures 245 x 25 x 9' 1 1/2". PLate E/2 is a mezzo tint plate of engine and engine-boiler layout. a real artwork piece.
While not likely to be at your local public library, certainly a university library will have this major work on shipbuilding. Perhaps you could use a camera or they might have a large photocopy setup.