Run time on a .15 Medallion

Don't contribute size for speed, the Medallion is a fairly low powered engine. It certainly would be quite faster with a modern .15. Your prop choice is also going to impact the plane's speed. I would insure that the engine is firmly mounted to the fuselage. No washers under the lug for engine offset as this can cause a lot of vibration. A old oil soaked fuselage is also a recipe for similar problems. The Medallion doesn't offer much of a footprint. I prefer to have at a minimum aluminum pads under the engine lugs and also on the inboard side of the fuse to insure a secure footing. Vibrations can create fuel foaming making the engine run leaner making it difficult to acquire a needle setting. If your really concerned about speed, put the prop on backwards and fly it as this will slow it down immensely.