I just looked at one of my 66s. You can access the thread guide from the faceplate. With the faceplate removed, you can see the other end of the thread guide. So, it is indeed just a force fit.
Just throwing out a bunch of ideas...
Can you secure the machine and make a dimple in the broken thread guide piece that if left? Then use a Dremel to drill it out? Protect the head. If you think you can, instead of drilling, try to tap it out. Suggest you keep the head on its back, so if the piece does come out, it is not lost and end up inside the head!
There does not seem to be enough of the thread guide on the inside to make an effort to remove it from that side. A long shot!!! But, if you protect the head and place a flat object on the inside
ON the thread guide, and on the outside
near, but not on the thread guide, you may be able to use a pliers without teeth to push it from the inside out.
Is there any way you can get a flat piece of metal on the inside, that covers the thread guide, and get a small clamp on each side of the plate , and tighten each one equally until the thread guide moves out the front enough to grab ahold of it? ( make sure you protect outside of the head hear also.
I honestly don't know which is the best way.
I realize I threw a lot out to you. Some may not be feasible. You will need to determine which way to go.
Any thoughts to help guys?