A few recommendations:
Buy a factory service manual.
http://www.repairmanuals.com
Buy this book:
http://www.amazon.com/Motocross-Off..._bbs_sr_1/103-9499230-6109454?ie=UTF8&s=books
The reed valve is between the carb and the cylinder.
From Eric Gorr:
"REED VALVE
Think of a reed valve like a carburetor, bigger valves with large flow-areas work best for high rpm power bands. In general, reed valves with six or more petals are used for high rpm engines. Reed valves with four petals are used for dirt bikes that need strong low end and mid range power. There are three other factors to consider when choosing a reed valve. The angle of the reed valve, the type of reed material, and the petal thickness. The two common reed valve angles are 30 and 45 degrees. A 30-degree valve is designed for low to mid rpm and a 45 degree valve is designed for high rpm. There are two types of reed petal materials commonly used, carbon fiber and fiberglass. Carbon fiber reeds are lightweight but relatively stiff (spring tension) and designed to resist fluttering at high rpm. Fiberglass reeds have relatively low spring tension so they instantly respond to pressure that changes in the crankcase, however the low spring tension makes them flutter at high rpm thereby limiting the amount of power. Fiberglass reed petals are good for low to mid power bands and carbon fiber reeds are better for high rpm engines.
Boyesen Dual Stage reeds have a large thick base reed with a smaller thinner reed mounted on top. This setup widens the rpm range where the reed valve flows best. The thin reeds respond to low rpm and low frequency pressure pulses. The thick reeds respond to higher-pressure pulses and resist fluttering at high rpm. A Boyesen RAD valve is different than a traditional reed valve. Bikes with single rear shocks have off-set carbs. The RAD valve is designed to redistribute the gas flow to the crankcases evenly. A RAD valve will give an overall improvement to the power band. Polini of Italy makes a reed valve called the Supervalve. It features several mini sets of reeds positioned vertically instead of horizontally like conventional reed valves. These valves are excellent for enduro riding because of improved throttle response. In tests on an inertia chassis dyno show the Supervalve to be superior when power shifting. However these valves don't generate greater peak power than conventional reed valves. Supervalves are imported to America and sold by Moto Italia in Maine."
A pipe and silencer will help a bit. The reed valve you talk about may help, but from what I've read, in some cases they can be difficult to jet properly.
I had my 2002 RM 125 overbored to 144cc's. That gives about a 20% HP increase for $500.00 - and that's amazing compared to stock. However, while more power will help in a few situations on the track, real, actual speed on an MX track is made by YOU - corner speed, how well you "keep it on the pipe", etc. . .
Porting and head work would help more than a pipe and silencer for about $250.00. The porting is included with the $500.00 big bore kit I got.
Finally, suspension work will also make you faster - you can only go as fast as the suspension will allow you. That's what I'm planning to have done this winter.
As Eric says, Speed is 95% rider and 5% bike. Better make that 5% count.