There are a TON of options out there if you want to do an R' up right. CP is making serveral designs for the R. Sphinx, Puma, Saber, ESR, Pro X, LRD, BDT, they all support the R still. It costs a lil' more to build HP with an R in my experience but the handles are sweet for sure. Stock frames are fine if you find a good one and balance the crank with a Pumashine gusset kit. I built a BDT stock cyl, a McCoy Pro X 310, ESR 370, and a CP 431 Puma and they were all big fun. The 310 was the best all around bike I've ever been on. Take a look at some of the quads of the month on r-central, sweet machines. My link If you've never had one, you should buy one. The Banshee is awesome but comparing the R and Shee is like apples to oranges. They both kick ass in their own way.
My lil' 310 made 54HP and good TQ:
Stock Cyl 86 with 88 plastics:
310
370
Puma