I have both OS and ASP/Magnum four strokes, though .30 size.
As far as price/quality/power, i find the ASP as good as the OS, even better if i'm permitted to say so. This engine hasn't failed me while the OS has for several times.

For your airplane, i suggest a .91 if you want similar flight characteristics as your .61 engine. Don't go the route of the FL-70, those engines seem to have a bad reputation.
If you do decide for the OS, check out the prices for the Saitos. They are lighter and have excellent qualities with the price being similar to OS's, being a good bet.