Not knowing the specifics what is alleged…
These engines have serial numbers. That may be seen on pages 8 and 13 here.
Proper preventative maintenance typically happens based on records of hours of use, gallons of fuel consumed, and calendar time since previous work. As loss of power offshore in the Gulf can be fatal, one would expect regular scheduled maintenance, which would leave records of the same somewhere.
Another question deserving an answer is what happened to the engines which were replaced in each instance. There should be a considerable “core” value, who received it? These are two per boat, so even if there was a catastrophic failure in one, there should have been one in running condition when removed. These invoices show no core credit or allowance. Someone got it somewhere somehow.
Slabbed previously profiled the person that was identified as the DMR employee responsible for the fishing fleet and indeed his name appears on the Johnson Diesel invoice RFP linked above.