Of course you can legally take your chances buying a kit or drafting it yourself. You can also legally replace your own brakes and perform heart surgery on yourself. Nobody would suggest that you do. You may save a couple bucks today, but end up unintentionally burdening your family with tens of thousands in legal fees as they attempt to determine your wishes - or fight over ambiguities or inconsistencies in the document. Legally, you may attempt to draft your own will. However, if it is defective, it may be unenforceable.
Take your documents to a new lawyer, who can draft you a will and other documents that fit your current situation. There is no way that a will executed in 1979 still reflects your financial and family situation.
Of course you can make your own will - but the question is, do you really want to? No one is ever required to hire an attorney, even if you're on trial for murder.
For my opinion on those will kits, read my article below: