I have owned 3 Dart 18s and had this problem with one of the 2 i still have(lost one in Ft Lauderdale to a hurricane).The quick ugly but strong repair we did so we could sail almost immediately and not wait & wait for parts (screws are metric) from Windsport in U.K. was to fashion new much stronger heavier chain plates/tangs and drill self tapping screws thru the fibreglass directly next to what was left of tang that failed. Very ugly but the port forward stay is now stronger then stock and I trust this repair for extreme sailing. I would never recommend to cut a hole in the side of the outside hull! i have had to replace entire rear transom and make large repair to the bottom of Dart 18 hulls where damage was thru & thru; and to a dozen other cats. Perhaps you could drill directly above area where tang is located(glas is thick there) and center the cut in top of hull ,and install an American made hatch( i have replaced both types of stock hatches on Dart 18s and find them not as watertite as Nacra hatch replacements). i never got the hang of sending/posting photos on this site, if u care to email me i will send photos Dart 18 on Fire Island where repair was made.