I have a Lady Sarah fitted with a Saito V4 which is a neat little engine but no longer made sp you would have to look on Ebay. Of the engines which are still available I think you could drive her well with the SVS engine from Tony Green Steam Models. They are available with either 8mm or 10mm bore and are 12mm stroke. They are double acting and self starting forward and reverse with a built in control valve so you could get away with just a 2 channel radio. The website is currently being redone but you can email them at firstname.lastname@example.org
or ring them on 01522 681989. A 6" x 3" boiler would be ample and will easily fit in the Sarah hull. I bought mine of Ebay but the workmanship was so bad I built new cabins from the Miranda plans. Photos of the SVS engine and my boat are attached I hope. Hope this helps, Ian.