I was not sure at first, but I think this question migrated from MO to MSE has been underestimated, and would receive better answers from MO users. It is related to another question (which was well voted and answered to satisfaction), except it is about a non-compact manifold (the line), which makes that answer not applicable in this case. I am not sure what to do to fix this, I will give some pointers to back my claim as an MSE answer, but it would be a shame that MO fail to play its role here.
There is (to my knowledge) no method to un-migrate a question. One can do things to achieve roughly the same effect but it would be rather messy.
A moderator could unlock and reopen the version here. Then there would be two version; the improvement would need to be copied over form the other site.
One could close the question on the other site, then reopen the unlocked version here (likely improvements would also have to be copied manually).
Both seem not really good options, which is why one should not migrate questions that seem in need of improvement.