Re: " Cost is not a issue, but speed and reliability is." < I have read that the Shoestring is a very fast plane even with a standard 60 engine such as an OS 60 so I wouldn't be trying to overpower it. If it was me I don't want to get into 90 size engines but I like the exrtra punch of the 75 engines. They don't over heat like a lot of the 90 engines if they are mounted in cowl and they are easier on fuel. The best engine I have found in that size is the Tower Hobbies 75. There is a pitts muffler available for the engine as well if the plane needs it.[8D]