'If that doesn't happen' is the right form.

'Won't' comes from 'will not', so applies a person deciding not to do something. 'Doesn't' comes from 'do', so just describes a circumstance.

You might use 'if you won't' or 'if it doesn't' but not 'if it won't.'

'If that doesn't happen' is the right form.

'Won't' comes from 'will not', so applies a person deciding not to do something. 'Doesn't' comes from 'do', so just describes a circumstance.

You might use 'if you won't' or 'if it doesn't' but not 'if it won't.'

Edit

Just realised it is more complex.

'Will it happen' and 'it won't happen' and 'it will happen' are all fine, but they never get combined with 'if'.

So

will it happen?
if it doesn't, we'll be fine.