I have had 900's and know a bit about them....
The engine of the 900 weighs about 16 lbs more than a small block CFi.
They do make a lot of torque but the weight and the gyroscopic effect of the crank make the sled much less nimble than its smaller brethren.
As far as shops to build your custom sled from start to finish (not in order)...
Air Design Customs in Boise, Lance Robinson builder
FasTrax Motorsports in Puyallup
My dream sled.... as long as were dreaming
2009 Assault as base
Struthers 860 Race gas, twin pipes but into a silencer (not stingers)...I like em quiet.
Full Indy Specialties built bottom end (bearings, snap ring, blue printing)
Full venting kit
Struthers Clutching
CMX belt drive system
FTX rear cooler add on
Titanium Holz front suspension, stock width.
COBE spindles
Fox Float EVOL 2's if they come out (EVOL, non rebound adjustable)
Sly dog skis with Shaper 4" carbides.
Mitch Evans Carbon fiber hood
Trail tech halogen lighting "frenched" into the hood by Mitch
All steel brackets in bulkhead replaced with high strength aluminum (eg. silencer mount, tunnel braces, drop brackets, shock tower and cross bar)
Custom Power Addiction light weight tunnel sides (Type 3 hard black ano)
Bulkhead and tunnel fully black hard coat anodized (type 3)
Titanium Holz Alpha-X skid (you would have to do a lot of sweet talkin to get this)
Straight, non tipped, Ice Age Rails
Hyperfax
Titanium bolts for the chassis and engine
Nextech Carbon upper and lower idler wheels with NTN bearings
Between the lines designs slide kicks scratchers.
Trail Tanks clear, high cap tank with integrated lightweight seat (skins protective gear)
Camo Extreme 146 x 2.5", 2.86" pitch track (hope to be out in August)
Light weight billet steering bellcranks
Light weight Aluminum steering hoop
Wild Child Cromo steering post
Ti handlebars (no riser, built to correct height), Willwood or Brembo master
RSI high power heaters
FTX throttle block
COBE front bumper in Ti (again... a lot of brown-nosing to get this part)
Full Skinz protective gear bag sets
A 420 lb, naturally aspirated 180HP sled ... oh yea.
All black with simple Assault graphics
One Bad Motha ultimate boondocking machine for a lighter rider.