I asked GR once how much HP a Mini fan sucks, running on his engine dyno.
He said he can't measure any difference with or without one.
(no need for a fan during dyno runs, engine is tank cooled).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

With front mounted radiators the the air flows cleanly and almost unimpeded to the radiator front, (which is good) and then becomes turbulent as it flows through the fins (which is good).
In a Mini, the radiator is 90 degrees to the natural flow. The air has to find its way around the alternator, water pump pulley and the shroud which is 30 or 40 ml thick. By the time the air flow reaches the radiator front it has to change direction, it is also restricted and very turbulent. All very bad for cooling.
To make an electric fan work well in this situation-
1. Mount it behind the radiator with a 30-50 ml gap.
2 Make a shroud for it so it can only draw air through the entire face of the radiator.
3. Remove all obstruction from the front of the radiator. Which would be the mechanical fan and the shroud to give the face of the radiator the best exposure to the air flow with reduced turbulence.