If you're going to do it, why not go 144cc?
The 01-06 models need crankcase mods.
Check here:
http://www.eric-gorr.com/twostktech/bigbore_specs.htm
SUZUKI
1990-2000 RM150
FSB: 58.050
PISTON KIT: Wiseco #9203P5796
GASKET KIT: Cometic Stock kits each model year.
PORTING MODS: Match export valve pockets to outer edges, raise export to 28mm, turndown cyl. Base 1.25mm.
HEAD MODS: 58mm diameter, .5mm recess, squish 6mm @ 10 degree angle, blend 20 degree
VALVE MODS: Match
CRANKCASE MODS: Grind front side.
http://www.eric-gorr.com/services/cylrebuild.html
SUZUKI
2002-2005 RM85 (92-105cc) $400
1990-2005 RM125/150 (133-144cc) $475
1987-2005 RM/RMX250 (265cc) $500
(Minor crankcase mods required for piston clearance on overbores for RM85 and RM125s, send the whole engine and the mods are included in the prices listed above)