no, you still need to rewrite, re-optimize, etc. everything
it would make it much easier to be fully compatible, sure, but that doesn't make it trivial
furthermore part of it's (mostly internal) design are strongly influenced by C specific dev-UX aspects, so you wouldn't write them the same, so test for them (instead of integration tests) may not apply
which in general also means that you most likely would break some special purpose/usual user which do have "brittle" (not guaranteed) assumptions about SQLite
if you have code which very little if at all changes and has no major issues, don't rewrite it
but most of the new "external" things written around SQLite, alternative VFS impl. etc. tend to be at most partially written in C
no, you still need to rewrite, re-optimize, etc. everything
it would make it much easier to be fully compatible, sure, but that doesn't make it trivial
furthermore part of it's (mostly internal) design are strongly influenced by C specific dev-UX aspects, so you wouldn't write them the same, so test for them (instead of integration tests) may not apply
which in general also means that you most likely would break some special purpose/usual user which do have "brittle" (not guaranteed) assumptions about SQLite
if you have code which very little if at all changes and has no major issues, don't rewrite it
but most of the new "external" things written around SQLite, alternative VFS impl. etc. tend to be at most partially written in C