You picked the right bike to ice race. If you spend as much on the chassis as the motor, IT WILL REIGN SUPREME. You will find that these things are far less complicated then the rest of your four stroke stable.
As far as the motor, you are smart for going with the 4mm right out of the gate. Get a HotRods with the 115 rods, and have it speced and welded.
Then there is your cylinder work. If you are able to salvage the ones you currently have you should have them ported and have domes set up as well by whoever you choose to do the motor. You will also need supporting mods. Cool head for the domes, reeds, intake, carbs, pipes, proper clutch set up, timing plate, transmission cut(Cam ATV does a nice stock cut) and shifting mods. It should get you low to mid 70's on pump or even 80ish on race fuel.
Your motor set up will be really close to the dune stuff out there. It will give you enough grunt out of the hole and still pull the big straights.
There are a handful of guys that have Successful flat track and TT motors out there. Jim at Passion Racing has put trophies in to many peoples hands with his motors. Brandon at Wild Card Racing has some success stories it TT. Cam at Redline seems to be able to turn any thing in to gold and I believe he has done a few winners in his time....... Then the names you won't hear much on here. Kevin Gigot, or John Stalworth. Pikes Peak legends! They know the motor you need. Those and only those names have I seen in the winners circle at Flat Track, TT, or Ice Racing.
With the motor set up, you will need to really work on the chassis. You could do some budget +2 arms with blaster or 300ex shocks. A nice Long travel set up would be the best. Set it so the quad sits sagged down in the travel so that when the front end is lifting out of the turn the wheels will droop and still make contact and allow you to steer in stead of pushing to the out side. Steer clear of the Full Flight arms. There just isn't enough room to work with a sway bar on them.(ask Jesse) As for the shocks, make sure to get something with a remote resivoir in stead of the rezzy mounted to the shock. That way you will have clearance for the sway bar. Next point, a good sway bar is needed. 450's can run with out them, but the banshee needs one. Wacko2000 is selling a dura blue bar at a great price. My buddy might beat ya to it though. If you need to buy it later, Sidewayz is making a new bar or you can get one from Rath. Try to avoid the clamps on the arms. They do work, but the bar works better if mounted to an actual shock eye or welded tab. Out back, get a -2 swing arm and a wider axle. Make sure your stock shock is fresh so it adjusts properly and use a lowering link with greasable zerks. I would steer clear of the R6 shock unless you are super fat or having it resprung. Been there done that, let my words save you money and time.
If you are really serious I would suggest some sort of ZPS set up all the way around. That allows you to change the quad height with out messing with the shock rates. Tires, some front tires with a semi deep lug and a flat profile in the middle. The Mohawk stuff is cool looking and will be ok through tracks that aren't cleaned up often, but on clean ice the flatter tire profile prevails. Out back, Cheng Chin C826's.
Set the bike up to "drag race" the straights and use the sway bar for the turning. Try to not get to low either. It will kill weight transfer which you will need to get hooked up. The front arms should never be parallel to the ground. If you can, get the quad at the appropriate height and keep the engine cradle area parallel to flat ground.
Like I said in the other thread, pm my your number and call any time you like.