I'm not intentionally trying to be vague, but rather just trying to explain of the dynamics of quadplanes and how they differ from conventional plane configurations. I agree that you need to "start somewhere" and there's two ways to do this:
1) you get the FX61 airborne by whatever propulsion means and determine Carson's by completing some zero propulsion descent rate tests. One way is to power down from high speed flight and maintain exact altitude until stall, say using alt hold. (which BTW is not always recommended with wings that can go into a unrecoverable flat spin even from 3-4 mistakes high). Ideally the quadplane setup would already be installed for this to aid recovery, and give realistic drag.
2) you install a motor with significant power, so more than required for cruise, and go through a series of tests to determine the best endurance/range for each prop.
Note that even a 30g 2206 sized motor can produce over 2kg of thrust now a days, so there is no real benefit in having a undersized, lightweight motor that only does a specific thrust to weight ratio to save a few grams. The thrust to weight ratio also doesn't take into consideration at what airspeed the thrust is achieved. Hence why all high speed aircraft have variable pitch props. If your aircraft doesn't have the right amount of thrust at the desired cruise airspeed, you're essentially stuck in the "wrong gear". In comparison the Mozzie drivetrain has better than 1:1, but only once the prop is at enough airspeed where it has enough "traction" and doesn't stall. The efficiency defining factor with electric propulsion is the correct prop, that in hobby products are sometimes only 10% efficient at converting motor torque into thrust. This is where most gains can be achieved.
One place to start is to copy some other models like this one with 100km range:
According to the current draw data there the drivetrain power requirements are similar to the MT/Mozzie and he uses an even steeper pitch prop to achieve it with roughly the same MTOW. So the same motor prop from the Mozzie should be within the ballpark. Also note that even if the prop efficiency was high and around 10g/W that FX 61 is achieving that range with only around 400g of thrust at cruise, so 0.25. It's likely you'll need similar for a QP setup plus 10-20% more depending on how streamlined and small the QP setup is.
Hope this helps.