The 8002B is already on the board of the cube-player. I made note of what's on the board as this would be one of the first questions a hard-core DIY'er might ask since the audio amplifier capability governs the choice/range of speaker Watts, speaker Ohms, etc. A PDF datasheet is attached - I only include this as background. For your application, you can ignore the techno-babble. Just put in a larger 8 Ohm speaker that fits.
If you are seriously considering this, I will assist you in identifying suitable items. I am an eBay addict and don't mind waiting weeks/months for "free shipping" from Asia to save a buck. But if this is for Christmas 2020, I suppose you ought to consider domestic sources though prices are generally 2x-3x YMMV. I've noticed that more and more of the eBay electronic stuff relevant to OGR projects can be purchased from Amazon that qualify for the $25 free shipping and/or Prime.
Also, it helps to know how this will be used. That's what I mean by i's to dot, t's to cross. For example, will the car ever run on a track with DCS? If so, there's a 25-cent "DCS choke" that should be added. Except you can never buy just one DCS choke without incurring huge minimum qty or shipping overhead. How exactly do you want to turn it on/off...or maybe it always runs when on a powered track? Do you really NEED (or just WANT) volume adjustment? And so on. This is all part and parcel of the DIY adventure!