Interestingly, there is a reputed North Dakota '41 pickup on Evilbay now with a four cylinder in it, good pics of the motor and such. 271036931074
Serious study of the '28-48 parts book would be needed to fully shake out interchange, but looks to me like valve assemblies and pistons and timing gears are most of the interchange. Parts should be very easy to get due to the armies of N tractors still working or being restored. Head looks like a total no-go. Don't have a real count, but looks in the overhaul book pics like stud count is somewhere around 16 or 17. Pickup engines with tin pan and fuel pump are VERY scarce, tractor versions all over the place. The public had already figured out that the V8-60 was seriously underpowered and was thus not likely to jump on an even smaller engine in a truck, though the 60 was briefly looking good during WWII with rationing and 40 MPH limit.
Unless your flat head has 18 head bolts you are out of luck. Timing gears and lifters do interchange. An old shop owner here had one he drove until 2 years ago until he went to the resthome . He replaced the engine with a V8 he told me when he got the truck it said USN on the side.