Yep, all the Airfix 1/24th scale kits have engines, but it's still a bit small to be a display model in its own right, it would only be about 75mm in length. A larger scale, say 1/12th . 1/8th or 1/4 would look ok, and if it was just needed to look like a Merlin, and not have working parts, it should be fairly simple to make. It could be done in plastic card and tube, or a combination of plastic, wood and thin alluminium or brass sheet. Or you could do what a chap did in the 1970's, for the Model Engineering exhibition in London. He made a 1/4 scale, working Merlin, with a prop, and ran it two or three times per day at the show!