Steve, the Big thing with the polini is the pipe, the kit almost is secondary to the pipe, that's why I like polini so much now that I feel like I better understand them, you can make them do almost anything with the right pipe.
I would start the build around a crank pipe or a modified tecno estoril, or an MLM but really fuck all that and just get your crank pipe on order. Next ya gotta keep it cool, I (obviously) like the mopedfactory head that I sell though dos cycles, or you can buy direct from me (same price, dos just probably faster shipping) you will also need a retarding CDI. You can set the timing low with points like ken says but then your midrange sucks. I am running the parmakit box on an A55 stator but there is a really good thread on this just posted a couple weeks ago. I would say hpi mini rotor or parmakit box are the most well documented, but anything that retards reliably.
I'm running a modified 2 shoe clutch, but I'm finally starting to hit the limit on that, my bike is close to 60 now with the 18x40 and the clutch is hating life riding dubs. I am gonna try the pavo shoes for next season since I've seen Lee's bike out a few times and it fuckin rips.
I haven't tried a 4 petal with the bigger carb yet on the crankpipe but Ryan's and Nick's are fast AF. I think it might be necessary to truly break 60 but I like the simplicity and tuneability of the phbg 19 with the 2 petal. I have tried all the reeds and like the Athena rubber coated with a spacer under it to clear the casting. I think anything from a SHA to a TM24 will work good on a polini, but the phbg is just what I had laying around. I have a polini CP I've been meaning to throw on there, maybe next season.