Random other question on another bike.
(Sorry for piggybacking, but I didn't want to clutter up the Bicycle Mechanics subforum with multiple topics.)

Broken-down, low-end 24" youth mountain bike.

Triple chainring, but it has no front derailer at all, just some rusty scarring on the seat tube from where the old one was removed.

The seat tube measures 31.8mm.
Biggest ring has 42 teeth.
The bike is set up for it to be a top-pull design.

I'm having problems finding a compatible cheapo derailer to buy. The Shimano Tourney TZ-30 doesn't list the right clamp size, and according to the shimano tech docs, the TZ-31 is made for a 48-tooth ring.
Do I need something like a TX-50? Any other cheapo derailers that will fit & work?