PR21BS
PR do 2 different 5 ports and a 3 port at the moment. I got my PR21BS in Sept 09. They are a ripper little motor and quite competative. It is true a lot of the parts are made by Go but to PR's spec. Mine has about 6 gallons on it now and its fantastic. The break in process is not as hard as it used to be with the Gen 4 Go's. The metal appears to be different and the piston and sleeves are matched by hand. The crank is balanced and Silica filled/stuffed or what ever you like to call it, it runs ceramic bearings and has quite different port timing to the Go motors. Power delivery wise, it still has strong but not as vicious a bottom end, and hugely improved Mid and High end. Typically I get roughly 9 - 11 minute run times dependant on weather and pipe combo I run, I usually aim for 9 with the stock 8mm restrictor in the carb.
These guys are doing there research and working hard at building a great motor. As mine was essentially a final preproduction motor, they discovered they had an issue with the needles in the carb expanding a bit which meant you had to tune rich when cold and it would lean off once it warmed up. They however change the needle with a new metal and longer LSN and this has fixed the issue, this was caught before they went to full production. I have and use the new needles, its great now.
They are a very worth while motor to have a look at particularly as you get similar performance out of these as a lot of higher price engines. For about the $250 - $300 mark, you really can't go wrong. If you get a chance, give them a try. If you want to get really carried away, pop a V-Spec carb in it (did that whilst waiting for needles to arrive), lots of fun then and easier to get the magic 10 minute mark consistantly. I am still experimenting with mine to see what I can make it do.
I do recommend a PR/APlus 0801 pipe or possibly the good old 2047 with it.