Milgard casement windocrank assembly
Thanks for posting. I believe your operator is our 39-253LB. Just ensure the dimensions listed below match what you have. For a new cover and handle, see either the 39-375 or 39-451, depending on the style you prefer. For a new track slide piece, see the 39-529. For a new stud bracket, see the 39-546. Take a look at all these parts and let me know if you think they can work for you. Let me know if you have any questions.
Unfortunately some older versions of this operator have covers that reallly weren't meant to be removed without drilling out the rivets, as you've discovered. Newer versions of the operator are meant to work with cover and handle kits that 'snap' into place. Essentially this means that you can't reuse your old cover and handle with the new mechanism, unfortunately; at the same time, if everything I listed in the photos above matches what you have, then the 39-253LB is what you need and there's no need to take the cover off to check other details. Let me know if you have any other questions.
You should be able to reuse the original stud bracket, yes. It will be compatible with the new operator in the same way.
I'm not sure what the plastic slide piece is in this instance. Would it be possible to see a photo?
That would be the 39-529 guide. This is sold separately but you can probably reuse your original without issue. These just pop on and off.