That should be a pretty easy mod. Tone stack and PI/poweramp would pretty much stay stock.
The "BE" channel is just a modified JCM800 preamp.
The JBE channel is the BE channel with the MPSa06 diodes engaged. You can put the diodes on a switch or use a relay for BE/JBE switching. Just be aware of the volume drop when you engage the diodes. A second master vol is pretty much mandatory if you are going to make this foot-switchable.
The BE channel on the JJ has a few slightly different values in a few places vs the BE100.
The other difference vs the BE100 is the .0068uf depth cap value ( .0047uf parallel with a .0022uf ) on the JJ vs just the .0047uf on the BE100
I would recommend you do a depth pot vs the fixed 220k that Dave puts in his amps, and the NFB resistor should be changed from the 2204 values to 47k off the 4 ohm tap.
So if you have a stock 2203 type amp, the only things you have to figure out how to physically add into the chassis / turret board are the resistors, cap, and diodes off the cathode follower for the clipping, and the depth circuit. The rest of the changes can all physically go into the existing locations.
I just hope you are going to do this into a 2204 clone build on a turret board. I would not attempt this into a real Marshall 2203 with a PCB.
If you use Synergy coupling caps, silver mica for the snubbers and tone stack, and those little Nichicon .68uf electrolytics for the cathode bypass caps, it will sound
Don't be tempted to "improve" the design by using "better" film caps on the cathodes, or high voltage ceramic for the pf value caps. It won't sound like a Friedman.