I have to admit that I have not yet found the perfect solution, without interfering in a party's internal affairs.
My difficulty in making parties responsible for candidates' debts is that there is a danger of making them indebted in a different way, of putting the party in a position of appearing to give one candidate an advantage over another, if it were responsible for higher debts.
One of the solutions to the problem is the Quebec one, as I understand the current system. In a leadership race, if candidates have not paid by a certain time, they become responsible for the debt personally. Perhaps that is something that could be looked into. So, in Quebec, it is also my understanding that the party is not responsible for the debts of leadership candidates.