Time to look for alternate solutions?
I've not tried it and do not know the true fitting size but I would check on unscrewing the bubble counter as well as the brass fitting below it as it appears to be a separate part. Then if it is a standard size fitting, look for an adapter to make it a compression fitting to grip the tubing you use and then add an inline bubble counter and check valves as you see needed. One of the reasons I like to move as much stuff off the reg /tank top is that things do get broken when we have them all hard plumbed together. I really like to use the cheap plastic Fluval ($8?) from a local shop and place it and check valves off somewhere that it is less prone to being broken.
Sometimes the best way to repair the funky commercial stuff is to simply replace it, so now might also be a good time to look at replacing the cheap needle valve with a better quality like the Fabco NV-55-18, but that is always a budget question!!!