Not knowing the year model of the ZTR 424, I just Google'd a wiring diagram and found this :-
http://www.dixon-ztr.com/ddoc/DIXO/DIXO1986_USen/DIXO1986_USen_424_.pdf
As AVB suggests, provision of the Engine numbers for the 15.5hp can help identify the alternator information and, I would also search the information based on the engine numbers for the original 11hp unit so you can directly compare.
Knowing what machine the 15.5hp came from would also assist in looking at the wiring schematics.
Sounds like it is not as easy as a ordinary ride-on motor swap. I just swapped out a 17.5hp B&S with a 16hp Vanguard and existing wiring from the alternator (only two wires) was directly compatible.

Environmentally friendly recycling at it's best