I've been intending to MIDIfy my Raffin 31-note crank organ, and am
planning to use the in-line valves made by Ragtime Automatic Music Co.
These are small cylinders with a nipple at each end, so they can be
spliced into the tracker tubing while appearing as just a "goiter" or
swelling in the line.  No mounting required; each valve is supported by
its tubing.   No screws in the woodwork, no Tees, just goiters in the
tubing and a bundle of small wires.

Also they work with either vacuum or pressure, whereas pipe organ
valves of the Peterson type may leak when exposed to pressure on the
"wrong" side.

Disadvantages:  the Ragtime valves cost $15 each last I looked, and
may in fact be no longer in production, and their very small size makes
it more difficult to organize the wiring, which just hangs in mid-air.
I do recall members of the MMD who had used these valves and were
satisfied.  I haven't checked Ragtime's web site lately as to whether
these valves are still available and at what price; sorry.

For the electrical issue of converting MIDI input stream into individual
electric drives for the valves, it's hard to beat the little boards
made by John Wale of jw Enterprises in the UK.  These come in 32 and 64
note sizes, can be cascaded to get a wider scale, can be set to listen
to just one MIDI channel, and -- most important -- John can program
them with your organ's scale, so you don't waste output drives on the
notes missing from our less than fully chromatic crank organs.  So, one
32-note board, quite small, will drive my Raffin 31er.  It cost me
about US$ 80, including shipping across The Pond.

A friend has ordered and received enough 64-note boards to MIDIfy his
home-built 3-manual plus pedal pipe and electronic organ, plus its stop
combination actions.  He's found the boards work as intended.

I hope this helps someone, and that someone will inspire me to get off
my duff and finish my own organ MIDIfication.

Mike Knudsen