50 years ago, I fitted a '2A 948' cam and S exhaust to a friends 2nd hand 850 Mini Van, no other mods, and it did over 250,00 miles at 80MPH, five days a week from Bromsgrove to Bristol and back.
Right, so basically it's fine to drive fast a lot and I shouldn't really worry about it
The 70 year old 803cc A series was a quite satisfactory engine in a Morris Minor / A35.
The 65 year old upgrade to 948cc substantially beefed it up.
The reduction to 848 retained the beefyness, but reduced the strain on the crank because the stroke was reduced, and the Mini is lighter than the Minor,...........so, the 848 engine has a 60 year old well proven history and is under stressed.
In full race form, they could produce 80 BHP and be revved to well over 7,500 RPM between gear changes.
You will know when you have driven it too fast................................................you get a letter in the post.