Sajeev, thanks for looking into this question. About the time I submitted the question I did go ahead and adjust the valve lash using the Hanes manual, the very same same video you posted above and a video from Classic Mini DIY (a fantastic source). A good set of feeler gauges are a must and they are handy for other jobs as well. The car has been running well since.
Ooooo! Sounds like time for an engine swap! 🙂 It's never too late to put a nice V8 and a 5-speed in that Mini. I'm sure a 289-302 Ford will fit. maybe even a small block Chevy; not an LS, that's just not right, but a nice 283 or even a 327. Given the size restrictions, I'd probably opt for the Ford, even though I'm not really a Ford guy, they are a bit smaller in envelope that most other V8's for the swap. 🙂