Closing up chimney area

[jido]

Hello,
I opened up the area which was originally a chimney then a cupboard in my room. It was part cemented part suspended ply, I want to reclaim the floor area.

How do I close that so we can use it safely?


The house was built end of 19th century.

At the far end there is timber fixed to the brick wall with pegs:


At the near end there is a single header which goes in wall and is linked to joist with a knot:


However, between the far end and the middle area there is a header that does not look like it is attached to the wall and that concerns me.

Basically, it seems the tail joist is kept into place simply because it cannot go anywhere else - and maybe supported by the plaster of the room below.

Here is a close picture:


Can I just remove the fake header and extend the tail joists? What size timber do I need?

 

[jido]

Here is a new picture after some more clean-up:

 

[jido]

We decided to replace the two cut joists entirely. Hoping the plaster for the ceiling below won't fall if we proceed carefully.