You will likely need to get a custom made spacer for that application. Don’t get anything cast aluminum. And for a 1/2” spacer you’ll need a good set of studs, that’s too narrow for a spacer/adapter style spacer that has its own set of studs (those are usually at least 3/4” or 1”). You will want to have the proper studs, good thread engagement and the proper torque for a plain spacer that’s a 1/2” thick. Generally I don’t like to go past a 1/4” for a plain spacer, the spacer/adaptors with their own set of studs are a different story.
You might be able to make a 5x100mm spacer work, the wheels shouldn’t be used on the 5x4” pattern but a spacer would likely have lug holes large enough to work. But I seriously doubt you’d find anything that would be hubcentric for that combination, and the machining to adapt it probably wouldn’t be much cheaper than having a custom spacer made. And if you’re having machining done you might as well have it done custom so it’s exactly what you want.
That would definitely be a better plan, although it would require moving the spring perches.