You should implement the context from scratch.
Weld has a clearly (and carefully) defined split between API/SPI and impl. If a class is in the weld-api or weld-spi, you can be sure that we follow the rules around deprecation and api stability, and do not change these apis from one micro release to another, and deprecate between minors etc. If it's in weld-impl, you can expect it to change without warning or recourse!
You should be implementing AlterableContext as you are targeting CDI 1.1. This gives you a destroy callback, which may be useful for cleanup.
Ask Stuart Douglas about activation.
I would write @Transactional from scratch.
Use @Priority to activate by default. The JTA spec should tell you what value to use.