The trigger will function without the circlip.
Just for devilment I decided to make a new trigger out of some aluminium flat bar as the old plastic trigger would allow the rifle to fire with the safety on if any excess pressure was put on the trigger pull. It worked. Took a long time with files, drills, hacksaws, vice etc but it worked. Maybe not as aesthetically pleasing as the Macarri version but it works a treat. I used the old plastic trigger as a guide, just allowing more 'meat' on the upper part that functions with the safety slide. Time will tell how long it will last.
Cheers, Phil (too much time on my hands!)