Once again, major kudos to you for uncovering information about the city bus fleets on the neighbor islands. It's a dream for me to go to the Big Island, but one of my worries of being there is being stranded in the middle of nowhere with limited bus service.
I dug into their heavy duty fleet with the provided info. Their 2000 model Gillig Phantom Suburbans are 40-footers (designation D) equipped with the Detroit Diesel Series 50 (designation 20), which is either with an original or an EGR system. Their 2005 model Gillig Phantom Suburbans are 37-footers (designation C) equipped with Cummins ISL with EPA 2004 certification (designation 29). Since you didn't provide a VIN on the 2007 Gillig Phantom Suburbans, I'm not too sure if it is still a 37-footer, or if it bounced back up to being a 40-footer, or what their engines are. Everything else after that is up towards speculation on what their transmissions are.
Their 2010 model MCI D4000CLs have the Cummins ISM with EPA 2007 certification, rated at 410 hp. Lastly, their 2014 model ElDorado XHFs are 36-footers (designation E, conventional flooring) equipped with the Cummins ISL9 with a possible EPA 2013 certification (designation CL). With no VINs listed for the 2013, 2014, and 2015 model MCIs, I can only imagine them being equipped with the Cummins ISX11.9. Once again, their transmissions are unknown.
As for their historic fleet, I did recall seeing a photo of an ex-OTS 1993 TMC RTS-08 40-footer, rebranded with the Hele-On logo and maintaining the same OTS livery. It was posted by the NorCalBusFans group on their J-Album gallery, but said gallery expired along with all of the pictures. In addition, they didn't reupload those photos of interest to their Flickr account as of yet.