Read this with a grain of salt, since I have only read the Webcat sources once while preparing an Enhancement PR for Keyboard Shortcuts. Only llelectronics can give a definitive answer for this and I'd guess that he would have to see the implementation shipped with SFOS3 first to do that.

With that out of the way:
I think porting the most basic browser functions should be possible without too much of a problem (technical, not time-wise).
But (of course there's a "but") I've seen numerous optimizations and deeper integrations/options which either don't have a direct equivalent in Qt WebEngine or at least would have to be rewritten and, of course, tested. This is not a small task. One option may be to first strip them out to at least have a working, but comparatively incomplete, browser in a shorter amount of time.
Another would be a rewrite of every option that's still possible to do before releasing.

I'd guess that even the "short route" would still take quite a while.

Also, I haven't heard which incarnation of WebEngine will be included in SFOS3 – some useful features have been included in later versions. Either way, it's a more recent rendering engine, but to compare:
WebEngine from standard Qt 5.9.6 (I think I remember SFOS3 having Qt 5.9) is comparable to Chromium 56 from January 2017. Let's hope it's not older than that.

cheers!