quick search on the forum gave something you might be able to use. Conver windows server 2003 to vSphere 6.5
Basically they suggest to make the P2V with VMware Workstation as a target instead of ESXi. When you have it running in VMware Workstation I would think you can do an export to ESXi.
Tip, remember to download the VMware tools installer to the Windows 2003 before doing the P2V. This will make it easier for you when having to install drivers etc when its running as a VM.