Unless you're *certain* the spark sound comes from the motor, you should check your battery and phase connections on the controller. A poor connection would work ok at low currents, but could arc on higher currents from higher throttle demands.
You could have windings on the stator itself that have damaged insulation, and the spark could be it shorting to the laminations, or to other windings thru the laminations. Or inside the axle itself, depending on how much of the wire you have replaced.
It can even be a phase wire connection to the windings, sticking out enough to eventually rub off the insulation and short to the cover as it spins. The pics below are from an MXUS3K 450x, but the problem can happen in any motor:
The 35xx series of crystalytes are not very well made motors, in my experiences with an HSR3548. This is one example of a severe mechanical problem you can encounter, because of the axle design, for instance:
What you are seeing is the axle where the bearing sits, has become "compressed" or worn (not sure which). If it's compressed, it might be the electroplating they put on the axle deforming vs the actual axle steel underneath that (there is clearly a layer of copper colored metal, and a layer of shinier metal on top of that, my guess is nickel plating to reduce corrosion on the axle steel?). Why it deformed is likely the out-of-roundness of the side covers, which are so out of round that they cause unscrewing and ejection of the cover screws over days to weeks (with loctite!), even having tried at least a dozen different screws (different metals, hardnesses, shapes, heads, lengths, thread shapes/depths, etc). Sometimes they just snap the heads off instead of unscrewing, making the problem worse for the remaining usable screwholes.
This axle problem results in wheel wobble, whcih makes the problem itself worse as it wears at the axle more, and it can make the cover problem worse, and eventually even make the covers start rubbing on the wires and windings....
The covers are also not cast very well, with defects and voids. Might be hard to see in this pic
but you can see the two pretty large voids as dark spots near the edges, across from each other, in this one of the other cover
