Do these "rafter ties" tie anything together?
Although I have done what you are asking several times, I had an engineer design and stamp plans, making a structural change this big is a big deal, actually the first one was a repair for a Bone head that just cut them out, the roof sagged walls bulged he could not sell and the bank shorted it, after repairing and turning this property I did several more on the side that had low ceilings.
All those places were similar, 
Recently helped a friend with a 2 story same issues but he did not have the $ for the engineering stamp. Good thing because after we repaired it it survived a huge snow storm that caused my barn roof to collapse.   But again someone tried adding height with knowledge or experience and that’s why my friend hit a killer deal as I have in the past. So don’t change things without structural engineering review when they can cause massive problems including roof failure.