I can't help with part numbers as I just sold my LT160 and bagger for a ZTR. However, if you end up with the bagger make sure not to use bags in the grass catcher's when bagging.

I tried this thinking I could save time by just pulling the full bag of grass out when it got full. Instead all it did was clog up in the chute. I think the bags restricted the air flow to the catchers. Just my .02

Top: AM121573
Frame: AM129832BLE (hopper frame), M131709BLE (stays on tractor), and AM124653BLE (main support)
Bags: AM125269 (bag frame), AM135485 (bag)
Chute: AM145781 (non Pwr-flo)

I didn't see any one all-encompassing kit, but those are the big ticket items. Not sure whether those part numbers include all the nuts, bolts, washers, clips, etc.

I have the 7 bushel bagger for my GT225. It's nothing fantastic. It is considerably better than my neighbor's Cub Cadet (generic MTD) bagger system. But a non-powerflow collection system certainly has its limits. I recently mothballed my 42C and bagger in favor of a 44M Piranha. So far so good.
Don't forget the hi-lift blades or the collection system will be useless. Also, when you use it, tap the bag to the right periodically to keep tabs on its level. If you wait till the flow indicator indicates no-flow, the chute is already stuffed with clippings, and you'll have to unhook it and shake it to clear it out. As long as you never let the right side bag fill all the way up, you won't clog the chute.

In reference to the bags in bags, the airflow needs to vent out the mesh collection bags to work properly. If you stuff garbage bags inside them, you block the airflow. If it's not picking up the clippings with the standard bags, you can get hi-flow bags (the tradeoff being that small particles are no longer filterd by the bags). It was a good try, though.