I have a NIB 1982 61 FSR GS A-2 but do not have documentation on it. So, I have some question on it like collectors value, reliability, prop size and fuel. Any thoughts?

The geared O.S. engines came out in the early days of Giant Scale. They were intended to turn 14 x 8 and/or 15 x 6 props or so. You'd run them with a fuel that has 20% oil content, and 5%-15% nitro. Don't have an RPM number, because would depend upon the gear ratio. You can figure that out by setting the engine with the piston at top dead center and one blade of the prop vertical. Turn the engine over until the piston and prop both are at the top and lined up. You'll need to count how many times the pistion came to top dead center, and how many full revolutions turned by the propeller. You'll quickly get the ratio from there.