2
You can use Wire.pins(sda, scl); before IMU.begin();.
IMU.begin() then calls Wire.begin() without parameters.
1
The main problem is, that the module does not get any supply voltage. You connected the Unos 5V to the OLED boards 5V. But this pin doesn't seem to be connected on the OLED shield. It is marked with 5V there, because this board is a shield to be stacked onto the Wemos D1. When you do this, you have 5V on this pin from the Wemos and can access it on the OLED ...
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