I found this speaker recommended on the forums for the FTDX101MP. This speaker mounts without modification in the external SPS-101 Separate External Speaker and Power Supply for the 200 watt MP model. It seems a little crazy to pay $220 for the SP-101 accessory speaker and immediately gut it and put in a new speaker for $11 that is better than the original, but this one fits there too. The eham reviews confirm this. https://www.eham.net/reviews/view-product?id=14965

NOTE: It will not fit inside the FTDX101D transceiver cabinet. However, if you go to the thrift store and get a wooden speaker box or cheap boom box speaker cabinet, you can install this one in that external enclosure or build your own box. Get two, for both receivers in the FTDX101.

NOTE: The manual for the FTDX101 specifies a 4 to 8 ohm load. The one recommended on the forums is 8 ohms. The one I am recommending here is 4 ohms, and will give you higher volume output, with the other specs identical. I bought mine from Parts Express, about the same price as Mouser, but cheaper than Amazon. The Amazon version is 8 ohms and has less output because of the FTDX101 audio stage design. The FTDX101 has only 2.5 watts maximum into 4 ohms @ 10% THD, and requires a speaker of above average efficiency to be loud enough. 8 or 16 ohms is less volume.

Visaton R10S 4" Full-Range Speaker 4 Ohm
•https://www.parts-express.com/Visaton-R10S-4-Full-Range-Speaker-4-Ohm-292-595
•https://www.mouser.com/ProductDetail/Visaton/R-10-S-4-ohm?qs=juaAHce7PkWJ41lAKc3Mxw==

Here are the exceptional specs for this speaker. It is popular for small guitar amps and PA systems. It has a big magnet, 20 watt power rating, and an efficient 90 dB sound output. https://www.visaton.de/en/products/drivers/fullrange-systems/r-10-s-4-ohm

The frequency response is quite flat, with a little boost in the speech/presence rise range at 3 KHz. It should be great for most SSB, CW, and AM, if you don't expect anything below 150 Hz. This speaker is less than $11 plus shipping. The wiring lugs on this model may not fit the existing Yaesu plugs on one side. You can change the Yaesu plugs on the wires to match the speaker if needed. Here is a complete how to install video with a demo of the improved sound: https://www.youtube.com/watch?v=RH1NJQbrH4k

View the nice flat frequency response for yourself, and decide:
•https://www.visaton.de/en/products/drivers/fullrange-systems/r-10-s-4-ohm
•https://www.visaton.de/sites/default/files/dd_product/r10s_4_fs.gif

Here's a cheaper ($190) speaker/enclosure option to the Yaesu SP-101 that DX Engineering sells that comes highly recommended for SSB. https://www.dxengineering.com/parts/pne-miranda-s The bhi is only $73 and gets good reviews. 4 inch driver with 1 inch tweeter. Same design as the one which has the amplifier in it. https://www.dxengineering.com/parts/bhd-extspk25#overview If you want an ACTIVE speaker with electronics like filters and a DSP noise reduction and 50-4,300 Hz response, bhi has this one for $310. https://www.dxengineering.com/parts/bhd-desktop-mkii#overview But your radio already has pretty good DSP and filters you can adjust.

Most amateur signals will not be below 150 Hz, unless you are doing ESSB or real hi fi AM. If you need more, you will need a more expensive, sophisticated studio monitor speaker, which won't fit in either the Yaesu SP-101 matching speaker ($220) or the FTDX101MP SPS-101 power supply/speaker. If you opt for an audiophile stereo speaker, try to keep it 4 ohms for more sound, and be aware that inefficient stereo speakers may require an active speaker or audio amplifier. The FTDX101 has only 2.5 watts into 4 ohms @ 10% THD. Good computer speakers might work, but beware of RF getting into them. I like to shop at thrift stores for speakers, and I get pretty good deals sometimes. https://wireless-girl.com/Projects/Receivers/BestSpeakers.html