Hello Mike,
You should take very careful note of Phil's (Weary) advice.
If you have been told that the LBSC designed valve gear is not worth improving, then those who gave you that advice don't know what they are talking about. KN Harris had a 'go' at redesigning the valve gear, as did Don Young for Reeves; neither solved the problems. Only Don Ashton's redesign gets it right. You can run all 4 on a computer valve gear simulator which will prove what I state.
On the fullsize loco the short combination lever has for the top two 'pins' a 'pin within a pin' to get the required proportions correct with no drop link. The 2 top pins are contained within the valve gear crosshead and guide.
LBSC used a pin below a pin for the 2 top pins, and therefore the combination lever proportions are incorrect.
He also dispensed with the valve crosshead and guide, which causes undue wear on the piston valves. LBSC 'Speedys' are notorious for piston valves that 'blow' after a bit of use, resulting in steam going straight up the exhaust with a 'woof 'woof' rather than a smart 'chuff' 'chuff'.
Roger Froud's example on the modeleng.proboards.com forum site as linked by Phil also has super duper piston valves for bronze cylinders, a redesigned smokebox draughting, and an improved boiler design, and much else besides, plus of course the Don Ashton designed valve gear.
It has the correct curves to the rear of the bunker, and the correct type of rear buffer beam – all defects with the LBSC drawings when compared to fullsize.
Cheers,
Julian