Will a 1927 engine with electric starter fit in my 1915 model without changing my firewall and radiator?

Yes, almost.

The left dogleg of the firewall may need a half inch of trimming to clear the starter flange and the floor board pedal spacing will be different. Even though the head is higher, the water outlet flange is at the same height. My son demonstrated that a Z-head will fit his '12 and match up to the radiator. If you keep the wide pedals, you may need to bend the brake lever.

I think that the changes of 1919 had no effect on the overall shape and size of the block, so I would expect the firewall to be OK. There may be some difference in the angles of the head and radiator water connections. I've never tried that combination, so I'm just guessing.

I concur with the emergency brake/forward pedal issue. My '23 Touring had a 26 or so eng/trans in it and the pedal & brake lever interfered with each other. The pedal just touched the lever in high gear.

The exact answer to the question is NO.

There is interference between the starter screws and the left hand fire wall. If you grind a little it can be reduced, but removing the starter without removing the firewall can be tough. The two nuts on the bolts holding the left hand firewall support are a little difficult to reach. I cheat and weld the nuts to the chassis....

Reading the thread again, there is a second problem, the water outlet is too long to fit the brass radiator, so it is necessary to use an earlier water outlet. Then how do you mount the fan, some 27 blocks do not have the fan arm adjusting screw???
There are work arounds but they take effort and time, certainly not a simple swap....

This is why my initial reply, speaking from personal experience, is "Yes, almost".

The water outlet has to be swapped and if there is a '26-7 front cover on the engine, there will be a mounting boss for the fan arm but no tensioner boss. This means the front cover should be swapped or you should really torque down hard on the pivot bolt or run without a fan as others have suggested on this forum numerous times.

I did mention in my first post that the firewall had to be trimmed slightly for the starter motor flange. I did not mention firewall brackets upon reassembly or that the adjacent hood latch is a bear to assemble.

If I were doing this swap, I would use a pre-26 hogshead and either use the earlier transmission or keep the four dip pan with a narrow brake band on a wide brake drum. This would minimize what is seen in the passenger compartment and also allow me the opportunity to swap the other minor parts involved.

My 16 coupelet had a 22 engine when I brought it home. To remove it I had to take off the firewall. Then I noticed due to the starter ear that whomever replaced the engine did not bolt the firewall brackets to the frame on the drivers side due to lack of clearance. I have pictures of the engine still in the car if you need to see for reference.