Larger wheels for '73 Europa TC

I've got the original 13" alloy wheels on my 1973 Europa TC.  I've found that 13" tires are becoming scarce (especially performance tires) and I'd like to get a lower profile tire if possible. I'm curious as to what solutions others have found to mount larger wheels since options for the stock bolt pattern are limited. Are larger diameter wheels available with the stock bolt pattern? Have folks had any luck finding adapters to change the bolt pattern to something more common? Are replacement hubs available with a more common bolt pattern? Any thoughts are welcome.

Mark Sr