Front derailleur change My son has a Boss Stealth mountain bike, The front derailleur has stopped working. It is a 3 cog Sunrun 42t (picture 1) which I am finding difficult to find.

The bike originally had Grip Shift changers and was changed to lever/brake combo about 6 weeks ago and worked fine so I don't think that has anything to do with the issue.

I have looked on Ebay and found so many different type I'm not sure which model I can replace it with. Picture 2 looks similar and has the correct teeth configuration. Picture 3 I'm not sure where the cable would attach.

Can anybody suggest a suitable replacement unit please
I think probably a Shimano Tourney top-pull derailleur that can accommodate the chainring sizes you have will work. You can see the basic specifications here to help pick out the correct model.

Front derailleurs are very basic mechanically and don't 'stop working' unless they actually physically break. It's possible you simply have the derailleur badly adjusted, or there is something wrong with the cable or housing.

Inspect the cable run from the shifter to the derailleur and make sure the housing is properly in the shifter body and all frame stops.

Disconnect the cable, tension the cable manually and work the shifter. Check the cable pull in and let out smoothly and does not hang up in the housing.

Verify the derailleur will swing from the smallest to largest chainring (it helps if you can pop the chain out of the cage).

If all that checks out, follow a front derailleur adjustment procedure. Park Tool has a good page and video with step-by-step guidance.