In N gauge terms we already have the Union Mills T9, and the chassis is a good basis for any 3D bodies even if the driving wheels may be a little big for some of the other Drummond variants.
I'm already planning to hack the UM T9 into a K10 and L11 with some N Brass Locos parts to replace the removed bits (one of my most ambitious projects to date). Those are both low-boiler types so it's not too much of a change, but the higher boiler types are unachievable except by scratchbuilding or 3D printing: S11 (the T9 wheels are a little large but not especially so if you're not a rivet-counter type - for me it's more about the right feeling than 100% scale accuracy), L12 (same driving wheels as the T9, so very appropriate), possibly a D15, and a C8 (which had a slightly shorter wheelbase than a T9 but again it's not a massive difference).
Colin at Union Mills told me to ring again in January about ordering a couple of T9s for my K10 and L11 projects, so that'll keep me busy for much of 2019, but it'll certainly be comforting to know that there could be further Drummond 4-4-0 projects to take up my time later in the year, or even next year.