If you reused heat shields and there are bubbles then I'd say replace the heat shields. Even if the heat shields haven't been touched in a while I'd say change them on the chance they were fitted the wrong way.

If you have the right tool you can try retorquing the prechamber locknuts. Maybe there's a sealant you can apply to the locknut. Or a very fine copper thread.

gsxr provided instructions for machining 603.96 prechambers to work with -17- and later heads. I don't know how precisely a reworked prechamber can seal since there's no compression ring between the top of the prechamber and the head in 603s are there is in 617s. FWIW I didn't see bubbling around the prechamber in my 603 which had machine shop reworked prechambers in a -22- head.

Sixto