Hi
So far,so good.
Any decent wheelbuilder should be able to supply spokes of the required length/gauge you want,motorcycle manufacturers don`t make their own spokes,in the same way they don`t make their own tyres or batteries.
Stainless spokes,even bicycle ones,are surprisingly expensive though.
If/when you do it,put some penetrating fluid on the nipples(Spoke
)well beforehand,you may well be able to re-use them(Decent ones are brass)
I wouldn`t run without an air filter myself,not for the sake of a missing cage.(You have spent VERY little on it SO far,as it is)
Have you tried getting back to the last owner to see if he has it tucked away?
It might be diplomatic NOT to tell him what a bargain you`ve got though
Good luck