Why don't you just go in there with a sawzall and cut everything out of your
way.
That way, when the roof falls in, you can have it rebuilt with rafters and
then you will have lots of room.
And that will only cost about $20,000.00.
--
JerryD(upstateNY)

If you're a half decent carpenter and you throw enough 2x4's at it,
you'll probably end up with something stronger than you started with.

If you use 1/4 bolts with fender washers and (or) #12 or #14 wood
screws and pre-drill the holes (and not nail anything together) you'll
have something stronger than the existing framework.

I bet your existing wood is full of splits because of the hack job
that is usually done when cutting rafters and pounding over-sized
nails in.

Do yourself a favor and make sure each and every rafter space is
ventilated out to your soffit overhang. Don't stuff the insulation in
there - let it breath.