I have a Four Star 60 with an OS .61 in it. It flies great, but on our hot summer days, and with our altitude here in Idaho, there are times when I really wish it had more power. The OS .90 is the same size and weight as the OS .61, and would be a great choice. I wish I had gone that route, and probably will in the future. If he is buying a new engine specifically for that model, why not spend a little more, and have a MUCH more versatile engine? Feel free to e-mail me with any questions regarding the Four Star 60, or the OS .61 I have in mine. I'll be glad to help!