I own a 1974 DT125. I have a Bassani exhaust and I plan on swapping out the factory exhaust. I’m going down to a13 tooth front sprocket and installing Boyesen reeds. What would be the recommended jetting? I plan on using this as a strictly woods bike. 

See what others recommend that might've done similar.
I tend to just go try it & see what gives.
If it feels right i'd just monitor the plug & carry on.
Woods riding is more on-off throttle than sustained big throttle opening.
You don't mention altering the air filter for more flow--if that's possible on your model.
Might be a less restrictive complete airbox setup out there.
Not sure about Boyesen reeds & not used them since the early days.
Probably carbon reeds out there that'd fit.

I'm not a fan of Boyesen reeds.  They feel like they give you better throttle response but they KILL horsepower.  Unless there is a good reason (like the stock reeds are bad), I won't use them.... 

(....but if we are racing and you are one of my competitors I'll buy you a set! )