Radio buttons in BYOs are named "[EXERCISE]_BYO_[#]_[#]." Here's your script altered for a BYO:
<script>
SSI_SetSelect('[% QuestionName() %]_1_1', true);
</script>
That will reset the question's response if a respondent submits the BYO then backs up to it. If you want to avoid that behavior, you'll want to use a script like this:
<script>
if (!SSI_GetValue('[% QuestionName() %]_1')) {
SSI_SetSelect('[% QuestionName() %]_1_1', true);
}
</script>