Honda gx630 performance

Might try Midwest super cub, Lakota Racing, Vogel manufacturing. I dont know if they have 630 performance stuff or not but will probably know where to get them if they exist.
You can make your own parts. A lot of folks do it. You got to learn a bit about sand-casting and lost-wax casting. You are going to need a metal smelter as well - can get expensive depending on the types of metal you intend to melt.