No Malmsey here either. I used Bual, Campari, and Booth's. The Bual Madeira was certainly sweet enough, but I'd cut down its proportion a little next time. A decent drink, but not as good as the very similar East India Negroni.
I had been making this with Del Maguey Vida, which is one of only two available locally, but managed to get a bottle of the Del Maguey Chichicapa that is called for in Joaquín Simó’s original recipe. The latter was marginally better, very similar. Both made for an excellent drink.
I usually make this with Evan Williams white label (Bottled in Bond). It is an inexpensive bourbon that I buy by the half US gallon and it is my go-to cocktail bourbon. The Maker's 46 definitely ups it by half a star. I think Maker's 46 cask strength would be even better, but you are then getting into Single Malt price range. Sam, himself called for a short, gentle shake, and I have been stirring recently.
It is definitely in my top 40, but I find it a little too sweet when using equal parts. 1 1/4, 1, and 3/4 works better for me. Or sometimes I'll replace the vermouth with a 50/50 mix of vermouth and Aperol.