This is what Wiki has to say:
Initial planning called for 20 aircraft to fly the mission, and 24 of the group's B-25B Mitchell bombers were diverted to the Mid-Continent Airlines modification center in Minneapolis, Minnesota. Modifications included (but were not restricted to):
Removal of the lower gun turret
Installation of de-icers and anti-icers
Steel blast plates mounted on the fuselage around the upper turret
Removal of the liaison radio set (a weight impediment)
Installation of three additional fuel tanks and support mounts in the bomb bay, crawlway and lower turret area to increase fuel capacity from 646 to 1,141 U.S. gallons (538–950 imp gal; 2,445–4,319 L)
Mock gun barrels installed in the tail cone, and
Replacement of their Norden bombsight with a makeshift aiming sight, devised by pilot Capt. C. Ross Greening and called the "Mark Twain".
ref Craven, Lieutenant Colonel Wesley Frank and Major James Lea Cate, series editors. "Drawing the Battle Line in the Pacific", Army Air Forces in World War II, Vol. I: Plans and Early Operations, January 1939 to August 1942. Chicago: University f Chicago Press, 1948, p439
Two bombers also had cameras mounted to record the results of bombing.
No mention of additional mods to the engines and, as you say time may have been a factor there. I would guess if it was possible they would have been modded.