I ended up cutting a disc out of an old fermenter lid and using that to block up the hole that was left when I removed the pump handle & mechanism. The existing rubber ring/washer seals fine and it was an easy task to fit a stainless steel valve in the centre of the removable disk.
I also cut another small disk and used that to block up the pressure release valve hole so I wouldn't accidentally spill or spray beer everywhere. Kept the dip tube, but trimmed a little off the bottom. Used one of those plastic picnic (brumby?) taps that others are using with about three feet of beva hose.
With the soda stream unit I needed to replace the rubber washer on the fitment, and found that a tap washer fits perfectly. I've removed the fitment from the soda stream case and had that just screwed onto the gas bottle making it a bit more compact. But, eventually the plastic where the lever/button pivots goes through broke so I ground back all the unnecessary plastic to make it smaller again, and fitted a press button to the top of the soda stream pressure release pin. For gas used same beva hose again with screw type hose clamps on each end with a Super Cheap inflater connection for a couple of bucks.
Actually, I found the support that usually sits inside the soda stream machine and holds the bottom of the drink bottle up made an excellent big press button once I drilled a small hole in the bottom so it would sit on the release pin properly.
If anyone is looking for stainless valves there is a guy in the UK called
Canmec who sells stainless valves on eBay in sets of 4 for AUD$20 and shipping was reasonably priced and quick. I know I could have gone hunting around town for them, and hear that places that specialise in 4WD tyres are most likely to have them, but all I had to do was sit in my chair and click a few buttons.