Depending on size of replacement stop and how many attachment holes are on it,I’d recommend ‘dry mounting’ over one of the existing holes ,then put masking tape on all four sides of stop ,you now have the ‘footprint’ ,using approx 240 grit paper ,remove all the paint back to the composite ,do the same on underside of new stop ,if it’s anodised alloy try and key up until anodising is off,maybe also key up the first few mm’s up the sides of the stop,find a v small ‘self tapping’ screw whose head nestles in stops countersunk hole ,now remove all tape ,blow off dust ,wipe all bonding surfaces with acetone or similar not meths or kerosen etc ,make sure everything is dry,hairdryer or hot air gun ,now remask borders of footprint allowing an extra 1 or 2 mm likewise if possible run masking tape around the new stop again allowing an extra mm or so if possible ,trim with a sharp blade so you have access to counter sunk hole ,now using an ‘epoxy structural adhesive’ such as the 3m range or Plexus [sorry you will have ALOT unused ]put the epoxy on both surfaces and into the old rivet hole ,squeeze them together also pushing the screw through the hole into the embedded epoxy clean up top of stop and screw head with acetone on ear bud or tightly rolled cloth,run wooden mixing stick or some such around the stop to clean up expressed epoxy ideally leaving a slight amount at interface ,tape over stop to chainstay so there’s a medicine of pressure ,been in composites repair/manufacture game for 35+ yrs and just done to process to a carbon frame at both chainstay and dual cable mounting on front end of downtube