The -300 is back in production courtesy of a freighter mob plus US defense force orders I think you'll find.
Yes Chocks, I'm fully aware of that - I'm still working 767 (along with the 747). Technically the -300 was never 'out' of production (most 767s produced over the last 10 years have been -300), although the 767-2C (aka USAF tanker) is -200 length. But there aren't any passenger 767s currently on order, and the FedEx and USAF tanker only work out to a plane or two a month (granted for many years).
Dominican - 787 is quoted ~20 less direct operating costs relative to a 767, roughly evenly split between the engines and the rest of the airplane (mainly weight) - a re-engine 767 could only get about half that unless they did a major overhaul to the wing.
We did a brief study of putting the GEnx on the 767 for the USAF tanker - but weight was a killer - the new big fan engines have much better fuel burn, but those big fans are HEAVY - a GEnx-2B (747-8) underwing weight is up well over a ton relative to the CF6-80C2 (less fuel offload for the typical mission).
787 is optimized for longer range - I'm wondering if a 767 re-engine would be better targeted as a 757 replacement. Mid range, less than 250 passengers, twin aisle makes for quicker turns relative to single aisle, aside from the engines it could be relatively cheap to build and if anyone cares anymore it's way more comfortable. The biggest issue I see is it would need a ~50k-55k engine, and there really isn't anything readily available in that thrust class that's 'new'.