Brexit again, should we have a second referendum?

They can only 'close the loophole' by repealing the Act which contains the provision for a referendum should there be any changes to the then current EU treaty. To do this they'll have to go through Parliament. Even if repeal is not opposed in the Commons it may well be opposed by the Lords, who don't have to maintain the tories in power...

And would it cause any sort of political crisis if they tried to rush it through?

But unfortunately, at the moment the EU Withdrawal Bill (containing the loophole) seems to going through Parliament with only minor amendments. We know the Libdems want a second referendum but I don’t think Labour have decided to support a second referendum yet.
Dominic Grieve does but will he be able to fight off the rest of the Tories, DUP etc even if he gets Labour support? The numbers are very tight.
Do the Lords actually have the power to block the Withdrawal Bill? They can delay it and amend it perhaps.

A second referendum might draw out some of those who didn't bother to vote or who found out after the day that their vote was based on nonsense. Those who now know that the 'red bus' promoted a lie that was apparently never meant to be taken seriously.
On the other hand would it see a change of mind of the (young) airhead who voted remain because leaving the EU would mean she couldn't go on holiday to Spain after the borders were closed!