Hey Shane,
{is my cue timely?}
i've just fininshed racing my RT1 at the 09 Conondale Nats. she's pretty stock~ish and a heavy bike by comparison to the competition but it ran around 8th place in the first 2 races and dnf~ed the 3rd with a broken throttle cable on the first uphill.
Chassis*
get some good shocks=yes
do the best you can with the forks as well.
thats pretty obvious info right.
do the engine lowering in the frame mod. mine has been lowered 2" and is a much better bike.
there heaps of info in this forum already in reguards to the frame mod.
it'll turn up if you go looking.
i use triple clamps from a later model DT with high rise bars and overall it's about as good as it'll be.
the 125 front hub is smaller and lighter.
remove the steel rear guard and make up a plastic inner guard to trim more weight.
weight*
just get rid of as much of it as you can.
engine*
pipe is a good one to have although i used my stocker for a while till i sorted my pipe.you can run into overheating issues with the stocker but it's rare.
34mm mikuni jetted to RT1MX spec
if you value you right leg,teeth and general well being, replace the points ignition with a cdi unit or the "kick~back" on start up will kill you..
the later big bike DT units will bolt straight in and a flywheel weight of 1650grams is spot on for tourque..
my porting and head are STD but are the next things i will be looking to do.
lube the crap out of your clutch cable and clutch worm/gear. fit lighter clutch springs if you can.
my aim has been to slim the bike down a little too as some of the "leg burner" pipes tend to make the RT1,s feel pretty fat. mine tucks in under the frame rails in the leg area and my seat is from an MX125a yammie.
i love my Rt1 but it's not for every track. if your on a pre~75 friendly track then cool but if the going gets tough then you'll need to ride it smart.
thats a heap of info and will take a little time but they're a fun bike. if you need any more info, feel free to give me a hoy and i'll see if i can help.
cheers,
Vandy.