Unfortunately it is VERY unusual for manufacturers to release service documentation for smaller UPS models, aside from a few leaked schematics for some older APC models, I've had no luck finding anything even for some of the bigger manufacturers (APC, Eaton, Alpha, etc). However, you've got nothing to lose by trying, have you already tried contacting the manufacturer?
That being said, with some knowledge about SMPS and UPS circuit theory and typical topologies, repairing them is not usually too difficult.
-All the MOSFETs should be replaced, even if they show no signs of physical and test ok.
-Thoroughly clean the PCB of any carbon from the burnt MOSFETs, as this can be conductive.
-Check the gate resistors for any open or out of tolerance resistors.
-Check gate drive circuitry for proper operation, this can be done by powering up the UPS from battery BEFORE installing the new MOSFETs, and using an oscilloscope to check for a gate drive waveform. A DSO is preferred since the UPS will most likely shut down very quickly once it detects a fault due to the missing MOSFETs. I usually connect the scope and set it up in single-shot capture, then power the UPS on. If everything is ok you will usually see a few cycles of switching waveform before the UPS faults off.
-Once verified that the gate drive is functioning properly, install the new MOSFETs and hope for the best!
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