actually W = N*m/s, so there is no conversion
Well... P=F*v
We have F from the original planes and we have v(speed from .dat)
So you could (at least according to math)
do the conversion - if you just take the max speed of the vehicle (converted from km/h to m/s). Then you'd have
some mathematically (but maybe not physically?) correct value.
Taking max speed for that equation would also kinda take care of the effect, that jet engines are least "effective" when v=0 and becoming more "efficient" with increasig speed, IIRC. So assuming that max power is reached with max speed might be easiest.
Just did that with approximate numbers for a B777 and came to ~105000kW - quite a lot.

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About the formula - I don't see a problem, you'd just have to re-arrange the formula, don't you?
so for v-max with a given weight it should be something like:
sqrt[(2500*power*gear) / maxweight] = max-speed