You are going to want something a little better than an ST90 to work with if you want to go a higher cc. You might as well get a Suzuki GT-185 like previously suggested. I bought one for my first motorcycle at $450 with only 5,500 miles on it. In fact I found one just like mine in my city, they aren't as rare as one would think. With how much you are going to spend on an engine you can pretty much have a whole motorcycle.
My Z50 with GPX150 is much scarier than the RD350 my buddy and I have been building, and thats with reed valves, 30mm carbs and expansion chambers. Your going to want a better platform or vertical engine design to beat his 120.
I have only been riding for a year and my garage is already full. After I could afford parts for my Z50 as a kid and got older it evolved into this. Vintage bikes are what I love.