Lots of the spelljammer ships come equipped with Ballista and Mangonel. Is there a way to create a custom item (that isn't a magical item) so these can be used with normal dice rolling actions? Otherwise my table constantly refers to the ship item page to remember the hit bonus and damage for these.

I assume there's some expensive gnomish contraption that effectively turns a ballista into a machine-gun crossbow. I don't fully understand what you're asking though. The weapon takes X amount of actions to load/aim, and then you roll an attack for it just like any other weapon.

you can simply create a custom weapon under the action tab by going to manage custom and in add new action select weapon 

add in the attack & details from Siege Equipment from chapter 8, Running the Game (i don't have spelljammer to check if they are the same) and set it as an attack action - this probably wont have the correct bonus to hit, just add it in your head

alternatively create a homebrew monster with all the details and add it in the extras tab. I've done this for a trebuchet & mangonel

Thanks! I'll try these ideas out. Much appreciated!

Thanks! I'll try these ideas out. sincerely appreciated

monkey mart