There's a reason why I asked if you were intending to split the case
Here's something else for your dreams.
I just added the DJ power curve for ease of reference.
The power output of the DJ is in part achieved by the cam and fuel injection. Running on carbs will not get you to the same power levels, and power is a mathematical calculation from torque.
To go to 2.1 you will need crank, pistons, mains, big ends as an absolute minimum, plus a bit of clearancing (Ian says rods as well).
You will need a good puller, and probably heat to remove / replace the dizzy drive to do one of the mains
Flywheel off and adjust the crank end float (you should probably do this regardless)
If you go to the 2.1 cam that'll cost money, use the DG one and output will probably be compromised..... but it may be more torquey
You'll probably have to make some jet adjustments to the carb, not difficult, but who knows what they are
After that there's no real difference in the rebuild, except you have a 2.1 instead of a 1.9
I have no idea if the 90hp is a good number or not, but similarly have no reason to challenge it. My DF/DG hybrid gives around 70HP, 10 more than a DF due to the DG heads, 8 less than a DG because of the DF cam...... but it only loses out on performance at the very top end because the cam gives low down torque.
I just did a simulation assuming your hybrid produces 53kW at 3500 rpm (80% of a DJ similar to the 90/112). The answer is 145Nm, looking at the graph a DG would be giving 138Nm
More money, better result, not a lot of additional effort