One issue is you are mixing up details at different 'layers' of
communication.
UDP is a transport layer protocol while ARP is a link layer.
WIth a direct connection between the PC and the FPGA, you don't really
HAVE a link layer, which is where a MAC address would matter.
The question comes how you will tell the PC stack how to 'address' the
packet to tell it to send it to the FPGA. That might require some hard
wiring in the stack to provide a routing to the FPGA. The FPGA likely
doesn't need to generate anything there, as it can be just fixed in the
driver to it.
As to a UDP port number, that would determine what the FPGA will do with
datagram.
Are ALL datagrams interpreted the same? if so, then it doesn't matter
what port you use. (maybe the datagram has enough details to define this).
Possibly, you want to be able to send datagrams to different 'ports' in
the FPGA to be processed for different purposes, and if you don't need
it now, you may just want to be using a fixed port so you can add others
in the future.