<VV> Knocking lifter, pushrod

[rbremner at dircon.co.uk]

My '65 140 has a knocking lifter and pushrod problem. My workshop found that the pushrod had burred at the lifter end as a result of what appears to be oil starvation (though it may not be this that's provoking the problem) and changed it. They checked the oil supply, which appeared ok, but this was done with the engine in-situ, so the investigation couldn't be called definitive. Anyway, 50 miles later, the knock was back.
Is anyone familiar with this problem, and is oil starvation a likely possible cause, or could it be something else? And, I need to get the car back to the shop and am wondering whether to drive it there gently - trailering it is not an option at the moment.
A detail that might be relevant - the engine has been apart at some time, and rebuilt with some Otto performance parts. 
The car is based in England, so I won't be able to take it to any US specialists of course. But the workshop I'm using is familiar with Corvairs. 
Thanks.
Richard
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