I can think of a few other possibilities for DS1742W replacements.
The first is to use the DS1744W. The DS1744W has more SRAM than is needed and is in a 28-pin DIP package. An adapter board could be designed which
grounds pulls high the unused address pins and, in the case of the TDS3000 series, moves the package out of the way of the chassis.
The DS1746W and DS1747W are also possible, but are even bigger with 32 pins. But it looks like they would still fit in the space available with an adapter board.
The DS1744WP could also be used similarly. The package is a module which is meant to to be surface mounted, and then the battery and crystal are snapped on top after reflow. It's more expensive than the DIP version, but maybe the battery cap could be removed and replaced after it was depleted. I don't know for sure since I've never actually seen one in person.
Alternatively, the chips are exposed on the DS1744WP module and they could be removed and remounted onto a smaller board that would have the same dimensions as the original DS1742W, with of course a crystal and battery.
Unfortunately, I wasn't able to find any matches for the RTC chip just by itself that has the same register layout as the DS174x series. That would be nice. I'm guessing the RTC chip has an SRAM controller on it which handles the power down and addressing, so probably any SRAM would do. Maybe ordering and examining a DS1744WP would reveal which RTC chip was being used.
Lastly, the access is slow enough for the -150 and -120 parts that maybe some really fast microcontroller could emulate the RTC and SRAM. But that would take a lot more work, and probably get ripped off instantly by the counterfeiters.
The DS1744W and later parts are currently available, at least for the time being, according to the EOL/NRND spreadsheet at Maxim:
https://www.maximintegrated.com/en/products/nla/index.mvpAny of these solutions would probably end up being in the $40-$50 range (at least).
EDIT: Fixed unused pins description.