You're going to be much better off replacing that system. If it's that old, you have a good chance of failure of the mechanism due to buildup or just mechanical wear (assuming it went through a test sequence regularly).
Hit up your local Lowe's (assuming you're in the US) and see if you can still find one of these leakSmart valves. No idea why Lowes was selling them off cheap with the rest of the Iris line. They're Waxman and have nothing to do in particular with Iris, other than the packaging said "works with Iris". Frankely, they're doing some crazy stuff. Someone got Caséta dimmers for $2.39 each. You can do well when the Lowe's staff haven't a clue what their discounting or why.
Anyway, someone posted that they were selling the already low priced valves for $40. This is a good valve (have one myself). It's Zigbee and has battery backup. Works with Hubitat by using a community driver. You can use any compatible leak sensor. I would have no problem recommending the Xiaomi leak sensors as long as you add repeaters. The new IKEA Trådfri outlets work great as Xiaomi repeaters. The Xiaomi use a community created driver too, but have been very solid.
Other sensors would be the Centralite that were sold as Iris. If you find any of those at Lowe's for a discount I would favor them over the Xiaomi, but both are good. You can also use any Z-Wave sensor that is compatible with Hubitat as well.
Setup is easy. Pair the valve, switch to the community driver, save preferences and click Configure. For the Xiaomi sensor it's basically the same. For the Centralite (Lowes Iris) sensor, the driver is built-in. It's the generic Zigbee Moisture sensor.
You then just setup Hubitat Safety Monitor to detect every water sensor with the flip of a switch and to close your valve and alert you. Very simple setup.
If you're looking for a more advanced system, professionally installed system to replace what you have, then you'll want to look at Flo. It doesn't rely on the leak occurring at just the right spot to hit your sensor. This system runs independent of Hubitat, but I'm pretty convinced there isn't a better system available unless it's an exact copy of their technology. They even cover damage if their system doesn't prevent a leak.
