If in any doubt check with a structural engineer, end of, he will give you a spec to form the opening to.
The rafters are the sloping timbers in a roof and have the tiles / slates on top of them. The joists or ceiling ties are horizontal and generally support the ceiling finish and are often used to take storage, although not always designed to do this.
A 50 year old roof is "probably" over specified as they weren't trying to cut costs as much in those days. Which normally means you can do a bit more with them without fear of failure.
I would have thought that if the two full length joists which are remaining are doubled up and bolted together, then the new timbers at right angles to these can be supported off them on joist hangers, and the cut joists supported on the new bridle.
The ceiling tie of a roof truss whether it is pre manufactured or not is generally used to tie the two rafters together and stop the roof from spreading out at low level.
Whilst the joists are unlikely to fail, unless seriously overloaded with storage, water tanks etc then the worst that tends to happen is excessive deflection at the mid point.
Last minute thought, does the opening have to go that way ? If it was oriented in the other direction then it would mean cutting less ceiling ties / joists.
