My problem is to find the equivalence between a IC engine with multi/speed transmission and a electric power train. Two 50 KW electric motors with a single ratio have the same performance of an IC engine with multi speed transmission?

Rather than looking for some equivalence formula - which will apply properly to only one specific combination of engine and motor - why not take it in two steps?

The transmission is used to keep the engine in the desired speed range, which in racing would be the best power range (not the way you drive on the street, or even recreationally). The spacing between gears is narrower in higher gears, so there's no single answer, but if you averaged the available horsepower (as shown on whatever power curve you can find online for one of these engines) over the range from whatever speed provides peak power down to whatever speed you would get when you upshifted from the peak power point, you would have an approximate available power. If you can't find gear ratios, I would roughly guess that the power after upshifting between upper gears might be 3/4 of the peak power, so the average could be 7/8 of the peak... so 150 hp or 110 kW. But use a real power curve for a specific engine and real transmission ratios to get a better idea.

The chosen motor must deliver this power over the operating speed range, not just at one optimal point. Ideally a PM AC or induction motor can run at the same maximum continuous output over a very broad range, so with the gear ratio chosen to put the maximum ground speed just within the motor's upper speed, the continuous motor rating is the available power at any race speed (less for the lowest speeds, where the motor is torque-limited). A brushed DC motor will have a much "peakier" power versus speed characteristic, so you can use a multi-speed transmission, or expect to run well off of peak power at most ground speeds.

Production EV motors are commonly rated at 80 kW to 100 kW, with some (for the lightest and/or cheapest cars) rated substantially lower. I suspect that you're looking to have more power than the typical (not Tesla) EV in a vehicle half the weight, with a substantially greater expectation of range... which won't likely add up. But that's why some real numbers are needed.

Remember that if your electric powertrain (including battery) is heavier than the gasoline powertrain (including fuel tank), you will need more power roughly in proportion to mass.