Oh, this could be really good! I've played with a couple emulators that had some proprietary restrictions, but still worked well for my needs. A good one free and clear would be really nice!
Here's one I wrote in Go, though I just noticed I never bothered putting on a license. Let me know if you want me to. https://github.com/lkesteloot/trs80
Why ask? If you don't mind how it's used, just make it MIT, 2- or 3-Clause BSD, or Apache 2.0