If your block is like my DPY engine then the stripped thread is/was 1/8″ BSP.
If so what about making a threaded sleeve out of 5/8″ A/F hexagon, (17mm A/F for the metric lovers), with a 1/8″ BSP thread on the inside and 1/4” BSP on the outside?
The block will have to be tapped out of course and the sleeve inserted with a drop of Loctite.
The sleeve needs to be carefully made as there is a barely 0.011″ each side between the crest of the 1/8″ BSP thread and the root of the 1/4” BSP thread.
If the use of hexagon is considered undesirable then a similar sleeve can be made without the hexagon. In this case a split 1/8” BSP plug with a tapered drawbolt through it to tighten it in the sleeve will have to be made. When the sleeve is screwed in flush and the Loctite given time to set the drawbolt can be slackened and unscrewed. In this way with the tap re-fitted the repair will be invisible, it’s just more work.