Taking apart 8mm video cassettes and properly re-assembling them can be a real PITA. The spring loaded meachanism can be hard to get back together properly, as you've discovered.
It's been more than a decade since I last had the joy of taking an 8mm tape apart, but I would suspect you could swap the spools into the new case and reassemble it. Or, you could possible re-spool the tape onto the new spools if you can remove the new tape.
The catch here is that you may run into the same assembly problems you had with your original cassette.
Have you tried running it in the player with the tape guard completely removed?
