I purchased a Mongoose Amasa Comp second hand many months ago and I have been told by a bike mechanic that the headset is the wrong size. When the front brake is pulled the steerer or the bearings slightly knock against the head tube.

I have been searching for a correct headset on the net but there are so many types and sizes im completely confused.

I searched the Mongoose web site and it says that the original head set was a Ritchey Logic Zero but I can't find what type of headset this is or what size it is.

Can anyone give me a link to the headset I need or an equivalent? Please.

This isn'y my bike but ity is the same if it helps.

https://www.moredirt.co.uk/ratebike.php?id=5234

I kinda doubt that you've got the wrong headset, there aren't that many combinations that'd fit together just a little bit wrong. OTOH there's a small wedge-shaped ring that goes in between the top race and the steerer tube and can easily get lost in a disassembly. If that happens you get what you describe.

Judging by the pic it looks like you might have one of the integrated/semi-integrated headsets. If you get yourself a vernier caliper and take a look at this link: https://www.sutherlandsbicycle.com/Chapter12.pdf, you should be able to figure out what it is that you've got.

There's a bunch of "Ritchey Logic Zero"s, with a few different dimensions to choose from. Might be that your best option is to hand the bike back to the mechanic and have him order a replacement for you.

Measure your head tube and steerer and then go to:
https://www.canecreek.com/headset-fit-finder