I could not find any official documentation for creating custom webparts on shareopoint 2013 and PI System.
But previously we had related conversation.
As discussed in here, I also recommend to use PI Vision adhoc function or PI WebAPI to get the data from PI System.
However, if you need to achieve it by custom webparts, this thread is also talk about custom webparts with SharePoint 2013.
It mentioned that PI Webpart 2013 SP2 upgrade caused error but I believe there are possibilities that custom webparts can run on SharePoint 2013. (I am not expert though)