Why not have a look at QuickJs. I assume your reason is to run js in the browser in an isolated environment for security reasons? I’ve found this article from figma to be an excellent starting point [0] Also useful to consider this benchmark [1] of js runtimes.
We’ve found quickjs-emscripten [2] to work really well thus far. You can see my implementation of it here [3] in Lowdefy. If still has a few rough edges, so would be really keen to make it better.
I’m interested to know, is spidermonkey more comparable to V8 than QuickJs?