Yep, you can swap the engine from the 115 into the 123 body. The blocks are the same with some minor differences. You may have to swap some of the exhaust/intake manifold studs, engine mount studs, engine and trans mounting brackets may have to be swapped but should fit the block and trans, AC bracket mount, and so on. Belt pulley may be different diameter, so use the one off the 123 to drive the accessories. The thermostat housing may have to be swapped, but the mounting studs are in the same position.

Injection pumps should be interchangeable. The linkage will have to be looked at.

While the engine is out I would suggest replacing leaking gaskets and oil seals while they are easy to get at.

A while back I rebuit a Euro 300D and replaced my US 300D engine and the above are some of the things I had to deal with. Otherwise, everything fit and the swap was easy in that regard.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.