There are numerous ways to address this.
You could source a qualified welder to weld the holes up and then have a machine shop surface the block. Cons: Time consuming and expensive.
You could drill the holes to the proper hole size, then tap them with a pipe thread tap, and then finally install a pipe thread plug with a quality thread sealant. Cons: Will most likely need to purchase the proper drill size, tap, plugs, and thread sealant. Also you run the risk of getting metal shavings in your cylinder head.
Simply install the KS Tuned G23 head plugs. Using the 3 supplied Oil-Resistant Soft Buna-N O-Rings per plug, these plugs will slide into place and offer years of leak free service. Cons: You may also find other useful items to purchase while browsing our site.
As you can see the most likely choice would be #3. Once you purchase and install our plugs, you will also notice that we offer one more benefit. Our plugs are threaded on the bottom. If you ever decide to change your cylinder head simply grab any M6 bolt thread it into the plug and pull out. This feature will allow you to insert and remove deep into the future without added expense.
