You may or may not have noticed, but OMEGAMON CICS V5.1 no longer comes with historical reporting. You have the CICS task history (ONDV) feature, but that is for looking at near term task history interactively within the 3270 interface of the tool. However, if you want to run performance reports, that's a different requirement. Long term, the direction is to get CICS Performance Analyzer (CICS PA). CICS PA has a myriad of reporting options.
But, if you still want to run the old OMEGAMON reporter, there is still a way. This technote shows a couple different ways to run the old reporter. You can still download the old code, or if you happen to still have the old V4.20 libraries around (specifically the TKANMOD) then so much the better.
Here's a link to the technote: