Modifying GDS 18-Watt for T-Rex Channel

[LedZepp007]

So, I (out of the blue) ordered a GDS 18-watt TMB combo kit (along with a Mojotone 1x12 cab and a Scumnico speaker). I'm very much looking forward to building it. I have no intentions of ever touching the normal channel, but at some point I may want to replace the TMB channel with a T-Rex channel (schematic attached). Realistically, how difficult would this be?

Thanks!