Is this your locomotive: MTH Texas "General" If so, it appears to be the same as the other "General" locomotives, and should be very similar to mine for the conversion.
With the MTH General, I had to mount the tach reader on the lower side of the motor, the bracket didn't come close to fitting. I also had to chop down the locomotive connector board to make it fit which required relocating the two diodes after the surgery. You can simply put them in-line with head shrink. You can actually cut quite a bit off the connector PCB after you relocate the diodes. Just make sure you don't create any shorts to the frame there, that's the kiss of death for the PS/2 boards many times.
Fitting the stuff in the tender was a squeeze as well, I couldn't use the speaker that came with the upgrade kit, too thick. However, a thinner one yielded decent sound, so that wasn't a major issue.
I recommend you put the weight back in! The locomotive needs all the weight you can give it. I used some fiberglass board pieces and glued the tach reader to the motor directly and dispensed with the holder for it.