One of the reasons I like the TT-02 is that the lower deck/chassis are inexpensive. Here are links/parts numbers for both the regular and hard lower decks.

Tamiya 51532 TT-02 lower deck (stock) - https://www.tamiyausa.com/shop/spare...02-lower-deck/
Tamiya 54926 TT-02 lower deck (hard) - https://www.tamiyausa.com/shop/optio...-lower-deck-3/

You can usually find/buy these lower decks on many online stores such as A-main, Amazon, Horizon Hobby, Tower Hobbies, etc. and it should be easy to order one through a local hobby store.

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As to swapping a 3mm screw for a self-tapping screw, yes I have done that on my TT-02. I was not careful when taking off/installing the spur gear cover on a TT-02 kit that was using 3mm screws (there are 4 screws that hold on the spur gear cover). Eventually I stripped one of the 3mm screw holes so I used a self-tapping screw. The self tapping screw held the cover well, but I made sure to not overtighten those screws. Eventually I replaced the whole chassis, but I felt it was a good semi-permanent/temporary fix.

I was racing this particular TT-02S kit in VTA, so I was changing gears/taking out the motor more than most people probably would on a TT-02. I also was using a Yeah Racing adjustable motor mount and didn't set gear mesh properly on 64 pitch gears (my fault/was in a hurry). I replaced the spur to continue racing that event, but eventually switched to 48 pitch gears which give a little more wiggle room if you don't get mesh perfect. I probably had changed gearing or removed the gear cover on that car at least a dozen times.

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I did a quick YouTube search and found the following video of an individual showing a good technique on how not to cross thread existing holes in plastic parts. This person was demonstrating on a plastic Tamiya CC chassis using self-tapping screws. Here is the link...
https://youtu.be/WYHwXix1dqo?feature=shared