The front fork broke instantly at the tip, just by testing the rotation of the motor in free air.
I think it would have been more accurate if you had said,
"The front fork broke instantly at the tip, just by tightening the axle nut!"If you look closely to the right of the crack in your photo below, you will see that the paint on the raised lip of the alloy casting has been damaged by the pressure applied by the edge of the tabbed torque washer.
The extreme sideways force generated at this point by correctly tightening the axle nuts
(even without over-tightening them) would have been sufficient to cause the brittle alloy casting to crack.
All of the clamping force generated by tightening the axle nut should have been applied uniformly to the recessed section of the dropout which should have simply been sandwiched between two washers. This would have ensured that a nice even compression force was exerted on the casting and should not have introduced any bending force whatsoever.
In your case, it is this bending force
(caused by incorrect fitting) that has caused the failure.
Even with correctly fitted washers
(see below) a torque arm would still have been needed to prevent the axle from twisting within the dropouts under high torque loads
(full power or full regen) causing the slot to be forced apart by the flats of the axle until the alloy casting eventually fails, allowing the wheel to fall out completely.
I strongly recommend that all ebike riders should carefully read Justin Lemire-Elmore's excellent torque arm article. 
This is my fork, do I need to change it ?
I would think those suspension forks should be OK to use provided a pair of suitable torque arms are fitted, but you might need to modify two of the supplied axle washers to fit properly inside the "C" shaped recesses on the outside of the fork dropouts to provide a nice flat surface for the torque arms to press against when the axle nut is tightened:
The outside diameter of the washers should be small enough to just fit inside the recess and should ideally be chamfered/radiused on the outer edge of the inner surface so that it does not apply any outward force at the outer edge of the washer.
If the washer is too large (or not chamfered or radiused to allow clearance for the fillet inside the recess) just tightening the axle nuts could exert enough pressure to force the dropouts apart causing the casting to crack:
So I repaired it as shown, but now I am afraid to install it on that bike, so I had to use an old bicycle with steel fork and welded torque bars.
The fork appears to be of pressure molded aluminum, and the thickness of the pipe trough the frame was thin...
I learned that pressure molded aluminum does not bend as steel, it just snaps off without no warning!
So question is, with my reinforcement of 7mm aluminum profile as torque bar on both sides, would it be wise to install the motor in front. The front damper had labels saying "not for downhill use or hard riding".
Cycle is of brand "Yosemite".
What do you suggest? I don't feel it's safe so maybe I should order a new rear motor? I feel like Golden Motor should have a stronger warning about using torque bars.
I would never trust 7mm aluminium as a torque arm.
Just imagine you're going to apply 90Nm of torque to a 10mm square section of steel bar by applying ~9.17kg of force at the end of a 1m long 10mm open ended spanner made of 7mm aluminium. The jaws of the spanner would simply open up under this load
(or break off if it was a cast aluminium spanner) and slip around the steel bar.
This should give you some idea as to why the steel axle can spin inside aluminium dropouts under load.
Anyway, as your forks are designed for disk brakes, they should easily be able to cope with a front hubmotor without any further problems, provided suitable
steel torque arms and properly seated washers are used to ensure the wheel is securely mounted to the forks.
The lateral forces exerted by a 1000W motor will be considerably less than the lateral force acting on the forks under heavy braking with a disk brake, so the steel stanchions
(upper fork tubes) should be more than adequate.
The only force that a hubmotor applies which standard forks are not designed to withstand is the torsional force of the axle, which is why torque arms should always be fitted to front forks
(steel or aluminium).
A conventional front wheel cannot apply any torque whatsoever from the axle to the fork dropouts as the ball bearings will not allow any torque to be transmitted
(well, not unless the wheel bearings seize up ).
Never underestimate the amount of axle torque that hub-motors produce
(especially 4kW Magic Pie's ).
Alan 
