Same machine source IP modification

[Insomniac]

My situation is that I want to modify all incoming UDP traffic going to a
specific port so that the source IP appears to be from the local network. I
completely realise this creates a problem as far as monitoring traffic goes,
but that is unimportant at this point. Being on the same machine, I have
found I cannot use the nat table, so SNAT and MASQUERADE are not available
to me. Any suggestions on a fast and efficient way to pull this off?