On my the V944 one of the servos stopped working. After investigation the culprit was identified as the FET driver chip on the main board. The defunct chip showed a slight deformation from the heat and had a brownish shimmer. The chip was labeled M02 and was found to be the double MOSFET (N-channel and P-channel) US6M2 from ROHM. The datasheet of the US6M2 can be found here.
The original chip has the size of 2mm x 1.7mm. A replacement ordered on AliExpress showed a slightly different dimension of 2mm x 1.25mm (SOT 363 package) but the helicopter was fully functional after the replacement.
The soldering was a little bit tricky as the servo connector had to be removed in order to reach the defect chip.
Here are some pictures:
The same FET driver chips are used for the V922 helicopter and probably for other WLToy models. Unlike the V944, the drivers of the servos of the V922 helicopter are not located on the main board but on each of the servo boards.