Yes you can replace the studs. Take the housing off, then put a pair of vice grips or a stud extractor on the stud and undo it. Put a 5/16" UNC bottom tap into the hole to clean it out before fitting a new stud.
I would take them all out and replace with Allen head bolts. This mod has several advantages:
1. You will not get the usual problem of the housing corroding onto the studs, as the act of undoing the bolts will crack off any corrosion between them.
2. These bolts are high tensile steel and less likely to break.
3. Whenever you take it apart it's easier to scrape the gasket surfaces clean.
If you want to make it look pretty, (BLING!) you can get some 5/16" UNC or Whitworth bolts in S/S instead from a boat shop- Bias Boating Warehouse are pretty cheap. Either 304 or 316 grade, won't matter. 5/16" W and 5/16" UNC have the same thread pitch, yes they do fit. Only difference is thread angle- 55* vs 60*, `dusn' matter' in this application, as Con the fruiterer would say...
Don't overtighten them. S/S bolts are not as strong as some people think.
If you fit either type of bolt use some aviation gasket cement on the threads, it makes 'em come out easy next time.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
