Argentine Hi power- front sight AWOL

I think Hi-Powers have a pressed-in barrel bushing, and that may be covering the area where you'd need to expand the sight base. I'm not sure I'd want to mess with the bushing.

You can, in some cases, have a dovetail cut (if the opening on the slide will allow it, and install a dovetailed front sight. That's a more elegant [and slightly more-costly] solution.

If you've got a gunsmith in the area with a good reputation, let the gunsmith guide you. He's probably had to deal with this issue before.

"...like a GI 1911..." Isn't just issue .45's. And they can go into the twilight zone too.
Anyway, Argentine BHP's use a dovetailed front sight. Press fit. That can be a DIY thing using a padded vise for the slide, a brass punch and a plastic mallet. Or buying a sight pusher. Right to left for installation. However, if you don't have that or have any doubts take it to a smithy.
Gunparts lists 'em at $16.95 for a used factory sight. $21.40 for a new, white dot, factory sight. $19.85 for a Capitan(a Mark III variant that features adjustable tangent rear sights) sight. Very high.