That is probably the same horn used in my Port of Lionel City B unit, and it is indeed pathetic! Sorry, no help here!
curiously, they list the two motors at different rpm's
I also noticed when looking at the supplement for the original version, that a foam pad was listed in the fuel tank. Perhaps that was the side to side buffer that was missing from yours. Anyway, you solved that.
Respectfully, some of you guys are really overthinking this. I’ve done this type of conversion many times, mostly on GP’s (4 or 5) and 2 RS3’s with no problems. The electronic e-units have been able to handle the extra motor with no problem. One thing you should do is inspect and/or clean/lube the power truck. I had two trucks that had hair (cat hair I assume) and steel wool. What a mess!! An alternative, if you want e-unit only, is Dallee Electronics. I’ve used their product many times with no issues.
I know the original e unit would work fine, and it did... until I pinched some wires putting the shell back on. It didn't survive that. Oops...