I'm not sure that they'd support the same functions, i.e. four comparators tied to four timers.

However, I doubt any SilLabs part would plug in in place of a '575 or any other through-hole part.

That's where this whole thing with using newer technology breaks down. If you have to build a one-of-a-kind device, which I do all the time, the SMT parts don't work, since you have to build a PCB just to be able to connect to them. Yes, I've tried those "adapters" but I've found them wanting. Seldom is there any place to put bypass members on the adapter, hence the supply leads get too long. Also, since most of those "adapters" don't have a solder mask, and NEVER have a good, thick, but accurate one, which would help with pin alignment, they're easy to mess up, along with the component on them. Finding adapters for 0.5 mm-pitch parts is difficult, and finding adapters for 0.65 mm-pitch parts is worse. Then, of course, there are those 0.8 mm-pitch parts ...

Without the datasheet, however, you'd have no chance at all of replacing the device, since you'd have to guess at how the comparators and timers would interact.

RE