There are a number of options. If you're using the same sumti twice in a row, {ri} is perfect for the job; they carry the referent, not the symbol. The counting rule for {ri} is a little weird: it refers to the most recent sumti which is fully closed, where we are counting in order of beginnings. So in {lo nu broda ko'a cu brode ri}, {ri} refers back to {ko'a}, as it is the most recently-begun sumti which is fully closed. This rule is slightly confusing because it does not mean that it refers back to the most recently-closed sumti, which in my example is {lo nu broda ko'a}.
The most flexible option is to perform variable assignment with {goi}, using the ko'V/fo'V series as your variables. A slightly more volatile but perhaps easier to follow option is to assign to letterals like {by}; this is technically less precise because the letteral variables are allowed to have their referent change from context, whereas ko'V/fo'V are really not allowed to do so.
A brief, useful, but imprecise way is to use letterals. {lo gerku} for example is often referred back to as {gy}. This obviously does not work for large sumti like abstractors.