128 DRAM only?
Version 12.4(19a) is the obvious candidate but this version was first published 2008 and there are more sinister vulnerabilities on this firmware. If there is no way of upgrading the DRAM then it's a no-brainer and risk getting hit with the NTP vulnerability than getting hit with the vulnerability found in 12.4(19a).
Do the recommended workarounds will be the safe bet.