Thanks!
I envisage the Il-31 as sort of a Su-7ised Il-28. Generally Ilyushin's answer to the Fitter family using an existing fuselage barrel for ease of production (in theory). So new 50-60ish degree sweep wings. New nose intake. Single engine in the fuselage replacing the bomb-bay. Something of the order of a Lyulka AL-7F which has similar smash to the Tumansky R-15 of MiG-25 fame - so supersonic performance via brute force rather than any finesse!
The Il-31M features revised intake geometry (principally auxiliary air inlets), uprated engine (AL-7M perhaps?) and a spine for better aerodynamics/more avionics.
I have a few export customers in mind and a plaything for the PVO coming up next. It may be interesting to speculate on an Ilyushin Fencer equivalent?? Maybe a Fencer-ised Il-54 with fuselage engines rather than podded in a revised aft fuselage. Sort of what the Il-102 was to the Il-40.
Sorry for the ramble.