Does anyone know if there is a SFP ONU EPON (not GPON) module that can work with Mikrotik router (RB760iGS aka Mikrotik hEX S) clientside and BDCOM OLT?
I want to get rid of ONU supplied to me by ISP.

Not tested but this module should work. I would check with the seller first if it allows editing parameters like MAC/LOID using telnet or web interface.

https://www.amazon.com/GEPON-Stick-Address-Supported-Attention/dp/B07QVPTG6M
https://dimiks.com/products/xpon/onu/epon-onu-sfp-stick

Fast forward 4 years, the links are dead and the text in them does not even hint towards what model it was or if it ever worked.
Does anyone know which SFP module works with mikrotik to replace the generic BDCOM GP1704 G(E)PON ONU?

Try this one: https://www.aliexpress.com/item/1005003515662920.html (xPON)