JVC PC-R11W Speaker Upgrade?

I recently picked up a PC-R11W, and the bass is very anemic, even with the bottom end boosted with the EQ.

They're listed as 12cm; what is the common size used to replace the 12cm woofers? 4.75", or will 5" woofers fit? I assume these need to be 4ohm.

I think this could be a nice sounding unit by upgrading the woofers.

Thanks!