Hi Jerry,
Your doing a geat job with that engine especially since you have limited facilities.
If I may advise you... it will pay you to lap the bores for the valves to get rid of any machining marks... this can be done using a length of nylon rod or even dowel rod turned down to a close fit in the machined bores... coat lightly with metal polish (Brasso or some such) and work it up and down the bores to produce a smooth even finish over the whole length of the bore.
Make the piston valves (spool valves) slightly tight in the lapped bores and then lap them in using the same metal polish... this will give you the best possible steam seal.
The slight differences in spool length will not prove to be much of an issue and the 19.1mm length will be fine... more important is the width of each spool... these must not be less than the port dia... a couple of thousandths greater would be perfect.
Take your time with these and all will be well.
PS... It was me who designed the valve gear for John's engine.
I look forward to seeing it running.
Best regard.
Sandy.