I have both a 1960’s Jensen model 75 and a model 70 . Can the firebox and boiler assembly simply be swapped between these two engines to alternate between dry fuel & electric heat ?
Yes and no!
The #75 has holes in the firebox to allow for draft for the flames. The #70's boiler has a heater blanket and asbestos sheets under that, which certainly should not be exposed to any open flame. I don't think this is the way you want to go about making both units operational.
Now if you are just talking about switching each entire boiler system to use with the same engine unit, then yes, they may be interchangeable in that manner, provided the mount spacings are compatible.
Stoker - I was considering the second option of switching the whole boiler/firebox/smokestack-base between the two engines so I could see how well the Style 75 runs on 110 Volts. [It appears that 6 nuts & screws and the steam line is all that needs to be removed?]
The ultimate goal was to set up one of the engines to drive a small set of little generators. If I understand correctly, in theory, the 75 has a bit more power, but the 70 has more RPM’s.
Save yourself the swapping and pick up a Jensen 25

Seems like a good enough excuse to add another to the collection
Save yourself the swapping and pick up a Jensen 25
Seems like a good enough excuse to add another to the collection 
Nick, this old experiment started when I tried make a basic light bulb shine with an old toy motor and a Jensen 35.
I do see that this steam hobby is full of chain reactions that oft result in the joy of having more engines
Save yourself the swapping and pick up a Jensen 25
Seems like a good enough excuse to add another to the collection 
Nick, this old experiment started when I tried make a basic light bulb shine with an old toy motor and a Jensen 35.
I do see that this steam hobby is full of chain reactions that oft result in the joy of having more engines 
Before you know it, you’ll be stacking them up like cordwood!😂
Holy cow Mike!
Looks like Jensen Heaven!
Looks like the Jensen factory!
From my experience the 70 actually has more power. I would use a newer flywheel with the groove and run the generator from that.