I've got an extra turbo that uses an internal wastegates to control boost. T-25 from an '98 Eclipse. I plan to build my own using
T-25 in a D16a CRX motor.
How can I adjust the internal wastegate to increase/decrease boost ?
Is it possible to remove the internal wastegate and use an external one instead ?

To lower the boost, you must either have a new spring put in the wastegate, or a new wastegate installed.
I believe t25's use a diaphram, and may not be adjustable. Altho the 7psi it pushes out stock + intercooler pressure drop should be good, you prolly don't want to go any lower then that.

To raise the boost, either get a boost controller, or bleed the boost away from the actuator, I use vacuum tee's sometimes. :D