Are you going to tear it completely down and do it, or are you going to drive it and fix it as you go? Since you're on a budget I'm assuming the latter?
If the 216 and 6 volt are working OK, I'd drive it till it goes, then replace it. I think a 235 is a common swap. You can decide at that time whether to keep the 6 volt system. They work fine if properly maintained. Otherwise a 12 volt conversion is not that difficult.
Good luck with your project and keep us posted.
