magento - How to handle Legacy code (Hydra code, complicated code), with a lot of customizations in PHP -

magento - How to handle Legacy code (Hydra code, complicated code), with a lot of customizations in PHP -

we have magento 1.4. have onepagecheckout extension installed. our checkout logic customized lot of times: there implemented taxation customization, next day delivery customization, payment methods customization, shipping methods customization, detecting , getting city/state according entered zip code, etc. result - have complicated logic, , if fixing 1 issue, getting new instead.

so, question is: best approach handle logic?

i appreciate answers.

php magento optimization checkout