The 16F628A has 2K words of program space.
The 16F818 only has 1K words.

You're going to need a bigger chip.
The Maidenhead routine with Floating point libraries will take >1.5K by itself.

You'll need at least 4K, but why not get one with 8K so you won't have the problem again later.
I'm sure you'll want to add a Morse Code transmitter and receiver some day.