Only try this if you are sure you understand the problem domain and are very comfortable dealing with all its pitfalls. Many times (especially when dealing with numeric code) 4500 of those 5000 lines are for dealing with special corner cases and obscure gotchas that people have spent month agonizing over how to fix and that has been battle tested by experts over years. Stuff which won't affect you 99% of the time, but when it does will leave you having to redo all their hard debugging work.