Nothing. There are plenty of words ending in "-ty,", but they are not really rhymes. Because of the accent on the first syllable, you would need a three-syllable rhyme--"-even…ty"--for a real rhyme, and there isn't one..
A better idea than trying for this rhyme, which is only going to be lame, would be to rearrange the wording in the verse so that "seventy" doesn't fall at the end of a line. (MORE)