Anyone know of a program that can select redrive ratio's, torque prop diameter etc.

What I want to do is say take an engine that will swing a 28x10 prop at 8000rpm and find out if it was fitted with say a 3:1 reduction what prop it would swing, thrust produced etc, and say select prop size and find out what reduction would be needed, or know the prop and reduction and what hp of engine would be needed.

Get the idea !

Don't know of a program for prop reducton systems. Pe Reivers has a hp/thrust program on his site.
Both Toni Clark and Mich Reeves have reducers. Using the stock engine as a base and what they claim with the reducers you could plot out a simple graph for a guesstimate of what your wanting.

This is easily calculated in a spread sheet program. Diameter changes to the square root of the ratio and pitch changes proportionally to the ratio for a given power and pitch speed. If you halve RPM, the prop diameter needs to increase by square root of 2 and pitch must be doubled to maintain the same engine load and pitch speed.

If you use [link=http://www.mvvs.nl]my spreadsheet[/link] download at the propeller section, you can first take the prop the engine likes best. Grab the power from the spread sheet. Then input the propeller you want to use, and juggle rpm until you find the same power.
divide the found rpm by the original rpm, and voila, there is your reduction.

That is a great little spreadsheet, difficult to know what pitch I want but It is for a very slow flying aeroplane (1/2 scale Bleriot) so dos'nt need any performance just fly it around the circuit.

Looks like a 3 to 3.5:1 reduction on a G62 will swing a 41" prop how is the prop constant figure worked out as it makes a big difference.
The prop I will be using will be probably hand made

seriously folks , most of those gear reduction units on a non piped engine are pretty much a waste of time
Why?
because they do NOT increase horsepower and they take additional power due to losss.
they simply trade off rpm and torque is multiplied by the ratio of the gearbox FWIW
as Pe noted. if you start with 5 hp -you still have 5 hp but minus gear losses. Which is not to be sneezed at.
a 41"prop of any type takes a lot of power to turn it and produce any thrust
1/2 scale Bleriot?
you will need 1/2 the power of the original Bleriot to fly it
In the large aerobatic models it is easily shown that as the size increases - the power requirements increase at a faster rate. (no formula needed to understand that!)
as size decreases , the need for weight reduction increases at a faster rate ( again no formula needed -it is all easily demonstrated in real time.)
The G62 are used in pairs on the Cri Cri and are equipped with tuned pipes
the setups on those planes is about as good as it gets in gear reduction equipped small engines.

prop constant depends on how the pitch is measured, blade surface (% of prop disk surface), and the airfoil used. More camber in the foil means more load.
The figures are derived from field measurements.

Dick, your post made it's way between my answer.
I do not completely agree.
In the old days of giant flight, the only way to get the thrust needed for a big plane was to use a reduction drive. WW1 scale fliers still use the technique that allows them to fly slowly and have more thrust per unit of weight.
It is a trade-off. You trade high rpm low thrust high speed for low rpm, high thrust low speed. The loss of the gear conversion is very much compensated by the gain/increase of flow speed bypass efficiency when your plane needs to fly at pitch speed. Look at it this way. A helicopter needs less power to lift it's weight than your 3D planes. A pure matter of extra reduction and larger prop disk, with reduced vortex flow losses. But at the price of lower speeds. It all is a trade-off.

Yes -I have made and used various reduction units
among em two Wankels on on center pylon- very smooth setup 2-1
the OS .61 internal geared setups with "hunting tooth" setups -very noisy with irritating warble due to changing exhaust/prop positions
very powerful-for their size -easily the best production setup we also piped em for 16000 rpm and ran 15x10 props!
The gasser setups - with poly V belts
- heavy clumsy etc..
anyway
I hope the guy gets enough power for the 41"prop.
The French adaptionof the G62 and a pipe works

I'm not tied to a G62, would like to stick to a single cylinder if possible but may go with a flat twin if I have to.