413 motorhome/truck/industrial motor "chat"

A friend had one in a test mule truck he got from Chrysler-- the thing would pull all day long & never get hot. I heard they also used them to power APU for the Air Force years ago. IMO it'd be fun to apply some modern technology to this old work horse & see just how much more power& endurance you could pull out of this thing. BTW, what's 1 of these things (in rebuildable condition) worth?