Reeds are not really an option unless you can get your hands on a later model top end such as an early 250 MX250 or DT3, but you will have to confirm that they have the same stud spacing as your 175 motor.
To get 100 mph there are a couple of variables. First is aerodynamic drag that quadruples as speed doubles, so anything you can do to reduce the surface area and the coefficient of drag would help. That includes lower bars and some sort of fairing to cut through the wind.
Next variable is rolling resistance, so brake drag has to be minimized and thin smooth tread tires help there as do new wheel bearings and a new chain.
Last is power. If you have enough power, shortcomings on the first two can be overcome but those bikes only made about 24 on a DT1/2 and 30 on a DT3 with larger carb. So you need more power AND better aerodynamics and preferably thinner tires to meet that objective.
There are a couple of DT based racers around in AHRMA with fully tuned motors where ports are optimized for top end power, squish bands are reduced to get the appropriate MSV (Mean Squish velocity) which needs to be higher on a street or low revving motor and has to be lower on a high revving race motor. Next we have a decent sized carb and matching inlet and a suitable exhaust that works with your porting.
Scott Clough races something similar and can point you in the right direction. To give you a more specific answer with less vague generalities, I would need a current port map, squish clearances, piston and combustion chamber dimensions and pipe dimensions to get started. Someone like Scott (SCR) or maybe Chuck Quenzler may already have that data.
To do it with a slightly less scientific/analytical approach, I'd start by looking at later model DT250 or DT175 motors to see what they have in common and what could be begged, borrowed or stolen from a later motor.
