Do you mean a propeller-driven bomber with the same payload and range as a Lancaster?
Well....
- The four Merlins would be replaced by two turboprops
- There would no longer be a navigator or flight engineer; they're superfluous
- Likely, there would be no gunners, either. There may be a weapon systems operator, who would manage the offensive and defensive systems (mostly ECM). Guns (and missiles) would be on the fighters.
- The weapons would be either laser-guided bombs or stand-off missiles
If you want four Merlins (why? It's 2020. Turboprops or — more likely — turbofans are much better), between rationalization of the spec and modern construction materials and methods and modern electronics, probably 20% in equipped empty weight. A general aviation aircraft can, with about 50 lb of electronics have better navigation equipment than existed during WWII. Another couple of hundred would add a multi-purpose radar. A bit more for EW equipment. Of course, bring that equipment mack to WWII and it would barely be recognized as electronics: where are the tubes?