You should be fine with a good copy made by a sheetmetal shop. Even the strictest AHJ I know wouldn't make you replace the entire panel just to get a new cover.

That said, we're replacing a GE panel this week. I'll have my guys save the cover and see if the model matches what you need. If so, you can have it. I think the one we're replacing is a 150 amp main breaker 20/30 style.