Hi John,
We briefly discussed this earlier this year on one of your other threads.
I think the simple answer is no, you cannot scale down the fullsize Stephensons valve gear but this must be qualified as it depends on the loco and arrangement of Stephensons and much else besides.
One of the classic 'cock ups' was Keith Wilson's 5"g 'Bulldog/Dukedog' valve gear – which is also relevant because it is pretty much the same arrangement you wish to copy. Direct drive Stephensons with loco links and slide valves below the inside cylinders. No way does the gear produce enough travel for the valve events required.
So unless you are vary careful and are aware of the pitfalls you could end up in the same mess Keith Wilson landed himself in.
Neil is absolutely correct that miniature locos require or ought to have greater travel of the valve than what would be scaled down from fullsize.
You start with the cylinder block and what width the steam ports and exhaust ports will be. (Only the width of the steam port is relevant for valve travel). Then how much lap do you want on the valve? Then in fullgear (say 78 or 79% cut off for a 2 cylinder loco) you can calculate the required valve travel required.
Then you have to consider the type of expansion link employed (launch type or loco links) and within reason you can predict the eccentric throw required.
Note with slide valves (outside admission) loco links are preferred with direct drive, and launch type links with indirect drive. In each case there will be a small suspension offset and a small amount of die block slip so the eccentric throw need only be a tad more than calculated.
If you have loco links with indirect drive, or launch type links with direct drive (for outside admission/slide valve) the suspension offset will be considerable to achieve equality of valve events and there will be considerable die block slip which also requires a greater eccentric throw and larger expansion link to compensate/accomodate the excessive die block slip.
Note Martin Evans used direct drive and launch links on Princess of Wales with slide valves and specified a totally inadequate amount of suspension offset.
Fowlers Fury mentioned LBSC's Pansy. I can state quite categorically that Pansy has a very poor version of Stephensons gear, but can easily be rectified.
John, you also have to consider the expansion link suspension and whether centrally suspended or suspended by the top of the link if loco links.
Anyway the first thing is to decide on the cylinder block and whether for a 5"g loco the size of Super Claud you have say 5/32" or 3/16" steam port width.
Cheers,
Julian