Mississippi Queens are humbucker sized P90s. A 'proper' P90 is not as wide and (I think) longer than a regular humbucker. I found MQs don't sound the same as a true P90, but you may think otherwise.
If you want P90s in a Les Paul, I'd buy one that has them as standard then upgrade them. Tokai/Vintage/Gibson/Epiphone etc all make LPs with P90s.