Can I legally ask my ex-husband to help pay for repairs on vehicles that I kept from the divorce that our boys use to go see him ?

You can legally ask, but it does not mean he has to agree.  Once property is awarded to a spouse, it is theirs to own for all purposes.  He should sign the title to transfer ownership to you only pursuant to the divorce decree. 

As far as using it for visitation, he might want to pay some to help reduce his travel time and expense.