Background info:
I bought this Ficus bonsai (pictured) back in Early 2022 and the main trunk was severed, with a smaller branch growing from the side which can be seen at the front. Unfortunately, due to a change in environment, this branch died, however, I have managed to revive the plant with smaller thin branches growing out of the side of the trunk.

My question:
Is there a way to get the main trunk to grow back e.g., by applying bonsai cut paste or is it too late?

Thank you in advance.

So you cut back the trunk below the graft? In any case the graft is gone now and the only way to get it to grow small leafs again is to make a new graft with a small leaf ficus. All the branches you have now is from the root stock. When the graft remains and are healthy these should be removed.

Now, in your situation I would prefere to shose a new leader and aim for.a bigger tree in proportion to the leaf size.

Yes, I cut it back to encourage new growth (it only had a small foliage as it was a baby ficus), but this propagated from underneath the graft as you can see. That's great, I will do that then. Thank you!