I have 4 Conions. 3 of them were stuck in play/record when I got them.
Here is how to get them in neutral position:
1. Press stop on the tape deck so the play key is not engaged anymore.
2. Take the back off.
3. Find the BIG flywheel of the tape and turn it. Even with no belts left on the deck, that should do the job.
You have to make sure to turn in the right direction! If you are looking at it from the back the direction to turn it is clockwise! If you turn it the wrong way, it might cause some damage if you are using too much force.
It is a little hard to reach the flywheel without taking the inner chassis of a Conion out. For having a temporary solution to not have it turned on all the time, it is the easiest to just carefully bend your fingers and turn the wheel mm by mm.
If you plan on replacing the belts anyways, you might as well take out the whole chassis. Be careful not to break off the head of the deck though if you remove the chassis while it is still engaged!
The deck of a C100 is relatively easy to work on, just take your time and be careful.
Hope this helps.