Super Game Boy Link Port
In addition to the clock speed issue, the Super Game Boy did not include a link port for multiplayer connectivity. It is possible to install one, but it requires fine-pitch soldering directly to several pins on the DMG CPU. If you have a steady hand and good equipment, here's how to go about doing it. Note that in order for this mod to work properly, you will also need to install the Super Game Boy Clock Mod.
First of all, you can purchase this mod board on my Tindie store: https://www.tindie.com/products/qwertymodo/super-gameboy-link-port/ The mod board comes with all of the necessary components pre-installed, as well as a pair of nylon standoffs to help securely mount the board in the cart shell. This mod board uses a GBA-style port because it is still possible to purchase them new without cannibalizing a Game Boy console just for this one connector. The GBA connector is fully backwards-compatible with the smaller-style port used in the Game Boy Color and Game Boy Pocket, as well as the official Super Game Boy 2. However, it is important to note that this mod is NOT compatible with actual Game Boy Advance cables (unless you modify them).
Once you have the mod board, you will want to begin cutting out a hole in the cartridge shell. I would suggest mounting it in the upper-right corner of the front half of the shell (upper-left if viewed from the inside). Start by placing the board between the two grooves in the shell as shown below, and mark the width of the connector.
Shell Cutting - USA Version
Make two cuts slightly inside the two grooves and begin to cut/file out a notch until the connector rests flat.
Once the notch is complete, position the board in place and firmly mount the hex standoffs to the case with hot glue or two-part plastic epoxy. Allow the glue to fully set.
Now begin cutting the other half of the hole in the back shell. I suggest using jewelers files, which can be purchased quite inexpensively online. With the board firmly mounted in place, you can easily test the fit as you work until you are able to close the shell.
Shell Cutting - JP/PAL Version
As with the USA version, use the ridges in the shell as a guide for the width of the connector and file down a flat surface until the connector rests flat. However, unlike the US version, the connector will be located on the RIGHT side of the front shell (as viewed from the inside) rather than the left. Also, it will be secured by gluing two screws head-down to the case to form threaded studs instead of hex standoffs. Note that you will also need to file a small notch in the ridge at the top edge of the shell. The rear shell cut follows essentially the same process as the US version.
Now that the board is mounted and the case hole is cut, it's time to wire it up. I recommend using good quality 30AWG Kynar wire, as several of the wires have to be soldered to adjacent pins on the CPU. The required solder points are shown below, including multiple options for Vcc (red) and Gnd (black). I recommend wiring Vcc to the through-hole via on the left below the "R3" text and Gnd to the left side of the bottom-right capacitor, or one of the through-hole vias next to it.
Soldering - No flex PCB
Start by soldering the wires to the CPU, leaving a good amount of length on each wire, and then gather them together and secure them to the top of the CPU to keep them from moving. Install the SGB back into the back half of the cart shell and cut the wires to length, leaving an extra inch or two, since the link port is mounted to the other half of the shell, you'll need a bit of slack to open and close the cart shell once everything is mounted. Once the wires are cut to length, you can remove both the SGB main board and the link port board in order to solder the wires to the link port board. I also chose to wrap a bit of tape around the wires to organize them into a nice clean bundle (I didn't use the Vcc via that I recommended above, I didn't find that one until later).
Soldering - With flex PCB
TO DO: This section needs updating, apologies for the poor photo quality.
The optional flex PCB makes installation MUCH easier. Simply align the flex board with the pins on the CPU and board capacitors, solder it down (using plenty of flux to avoid bridging), then use the included ribbon cable to attach easily to the link port board. The ribbon cable simply attaches straight through, for both the USA and JP/PAL installations, assuming you followed the mounting instructions here. On the USA version, extra care needs to be taken not to pinch the ribbon cable in the screw boss when closing up the shell. I will probably move the solder pads a bit to help avoid this in a future board revision.
All that's left now is to mount the boards back into the cart shell and close it all back up. This can be a little bit tricky if you didn't leave enough slack in the wires, but it shouldn't be too bad.