Technician reported variance from the MMEL during an engine repair. During a records check; it was discovered that the aircraft logbooks did not accurately reinfect the corrective action.

Narrative

On or around DATE; Aircraft X returned to ZZZ after completing survey work. It was noted that the LH engine had an oil leak near the alternator. After cleaning the engine and performing a visual inspection; a dye penetrant inspection was conducted which revealed a crack a little over an inch located on the upper crankcase behind the alternator. The aircraft was removed from normal survey use to await replacement of the engine with an overhauled engine that had previously been ordered as a spare. One of our IAs remembered a service bulletin regarding repair of cracked crankcases. The service bulletin was associated with AD 77-XX-XX XX. If the case could be repaired; then the aircraft could be used to survey storm damage near ZZZ until the replacement engine arrived. The crack was repaired IAW section (c)(1) of the AD. We then determined it would be better if the alternator was removed so that the repaired area would not be stressed. The POH; 91.205; and the TCDS for the aircraft and engine were checked and nothing was found that would prevent operation in this configuration. A blocking plate was installed; the alternator circuit breaker was pulled and collared; the field fuse was removed; and a new weight and balance was calculated. The surveys can only occur during the day; so it was decided to restrict the aircraft to daytime VFR since the POH required electrical load to be maintained at less than 80% of the functional alternator.After FAA Aviation Safety Inspectors visited our location to perform OJT with logbooks from some of our aircraft and asked about the repair on Aircraft X; additional research found that a Master Minimum Equipment List exists for this aircraft and did not allow this configuration. The aircraft was immediately grounded and NAME was notified of the mistake. The alternator and drive coupling were replaced with new units to restore the aircraft to its original configuration. The aircraft will not be put back into service until the replacement engine has been installed. Maintenance entries for the work performed were not placed into the logbooks or signed. It had been the practice of the company to work off discrepancy/corrective action work sheets and then create a handwritten draft of the maintenance entries before storing them digitally onto computer or iPad until the aircraft was preparing to leave Maintenance at ZZZ. Before the aircraft would return to survey work away from home base; all the maintenance entries would be printed; signed; and placing into the appropriate logbooks. This was done to condense/compile all the maintenance entries - all entries would be organized and entered at the same time. The maintenance entries for Aircraft X were updated on DATE1 after the mistake was discovered.The main factor involved is lack of assertiveness. After I was shown the POH and checked the TCDS for the engine and airframe; I relied on the judgement and knowledge of the IA. I had once checked a Master Minimum Equipment List for a King Air at my previous job but was not aware that they existed for the T310R. I should have gone beyond 91.205 to 91.213 which would have answered the question. In the future I will trust in my own judgement and ability to research the FARs.