There's at least two years of work in there as Fabrice Bellard's side project. This would mean about ~10-15 man-years of full-time work for a small team of experienced engineers, which is about what it took for jerryscript:

https://github.com/jerryscript-project/jerryscript

Granted, the targets aren't exactly the same (micro-controllers vs very small lib), but benchmark differences are impressive:

https://bellard.org/quickjs/bench.html