The Mamiya 645 existed in the times when 'flash dedication' was not common, particularly in medium format SLRs. there might only be one connector for flash ready signal, whose position varied by brand, but as only an indicator in viewfinder, it had no affect on general functionaliy.
The Mamiya 645 Pro-TL , manufactured from 1997 to 2006, is functionally similar to the 645 Pro but adds TTL flash metering.
So unless you own specifically the 645 Pro-TL (Pro TL shown only one the SIDE of the camera and there is a special TTL connector at the far front bottom of the left side panel), the front says 645 Pro'), you have no inherent flash compatibility issues...the hotshoe only has generic ground pin in the shoe, and trigger pin centrally located, so you can trigger any manual flash or any photosensor flash (commonly mistakenly called 'thyristor flash' as it does not read the scene brightness)
If you attempted to trigger a Mamiya-compatible TTL flash, the lack of 'quench' signal from the body to the flash would simply have full power output for all shots.
Today it seems difficult to find a new flash unit which is NOT DEDICATED to a specific brand of camera...a couple do exist, but most speedlight units today are brand-dedicated.
If a dedicated nTTL flash is used on a non-nTTL body, the flash would not get any pre-flash command from the body, and the camera would NOT send a nTLL power command to the flash a moment before shutter opens and trigger occurs...IOW, it should work fine on your 645 Pro.. No guarantee it would.
It may be better to find a not-camera-brand flash which was contemporary to 1980-1990, before there was as much dedication. A flash like Metz would be perfect.