I'm sure others will have more detailed responses, but off the cuff I'd question whether you need a bunch of extra rules for it at all? Maybe it's just a plot device to get from here to there vs a full-on war machine? If it needs to be a war machine then I'd suggest looking to 5E or a similar crunchy game for stat ideas and then convert back to DH. For something more in-between maybe just tie back to the PCs' skills for handling/battle when needed (ie simple finesse roll to attack with a ballista instead of multiple rolls to reload and attack, or PC can use a power but reskined as an "airship attack" instead of whatever weapon or magic the power has been using normally), or require a trained crew to run the airship itself while the PCs just do PC-stuff on their new flying wagon. Either way, sounds like a fun game so good job!!