I purchased a 2005 Civic EX D17A2.
I found someone had cut the cat out.
I'm replacing head, timing belt, water pump, head gasket.
Will I have to replace catalytic converter also?
Or can I screw sensor into a 34" pipe 3" long... Mount it to the body?
Will limp mode activate?

 

You won't get a limp mode but you will get a p0420 for no cat.

If you don't need to pass emissions and can live with the CEL being on all the time then send it.