Hi and welcome.
RVF400, Honda RS250 and ZXR250/400 are the "common" USD fork swaps, none are straight swaps though.
Ohlins and Maxton both do rear shocks but if your bike is an se or sp both the front and rear were once good units and someone like K-tech (but closer to you??) will rework them for very little money compared to what you'd spend on aftermarket stuff.
Yes, the stock card does work after the wiresplice, you lose the speedo, mileage reading, oil warning light and neutral light with the HRC card in, you still lose the oil and neutral lights with the Honda card back in and the wiresplice done. Mileage doesn't go up with the HRC card in.
Not the ignition but the mc21 flywheel offers more advanced timing. I would normally say do not use one with the HRC card full stop but as you've got a TZ I guess your pretty familiar with the way a 2 stroke works so I'll just say the '28 was further restricted by retarding the timing (compared to the '21). This can be "fixed" by either the HRC card or the '21 flywheel. Doing both could be disaster on normal 95 ron pump fuel.
The HRC card is the prefered way as it totally derestricts the ignition, removes the rev limiter etc.
The airbox can be modded with a couple of holes in the lid about 1" across.
Obviously the jetting will need checking.
You'll find with all this done and running well the standard gearing is too short. I've just got 16/39 for mine (std is 15/43) but not tried it yet as it died again.
There aren't many '28s about and even less people modding them so nothing about them gets "done to death" on here.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread