Are the Norton 961s OHV engines? I have no idea, never paid any attention
My recollection is that hydraulics lifters were pretty much universally used in vanilla pudding North American V8s but any high performance V8, such as the 340 Dodge or the KCode Ford 289 got solid lifters.
In the hotrod world one of the common performance mods is to do away with hydraulic lifters and go to a solid lifter setup with an appropriate cam profile.
So I'm not sure we should be trying to fit hydraulic lifters to these old hotrods.
A bit on the modern day fitment of hydraulic vs solid on an old Mopar 440.
https://www.google.ca/amp/s/www.hotrod.com/articles/mopp-0312-solid-vs-hydraulic-lifters/amp/
Glen
FWIW...all production V8 Mopars (Chrysler, Plymouth, Dodge) beginning in 1968 had hydraulic liters from the factory. That included all the high performance engines - 340, 440, and 426 Hemi as well as the "standard engines" - 318 and 383. I owned cars with all of those engines from new and worked on them literally all my life. I did tons of work on Ford/Chevy as well but "specialized" in Mopars. I also campaigned Mopars - a 1968 340 Barracuda, a 1971 440 Challenger, and a 1968 Hemi Barracuda in the SuperStock drag racing class. The 68 Hemi Barracuda was a special order, race only vehicle sold specifically as a drag racer. It was not street legal and only around 50 were made. NO, sadly, I didn't own it, just wrenched/drove it. An original will easily fetch a million bucks nowadays!
Of course,, the old classic car saying could apply, revised for the specific: "Mopar made 50 1968 Race Hemi Barracudas, of which 1412 survive."
I made up the specific numbers - but cloning them is pretty common.
Higher performance cams, still with hydraulic lifters were sold through Mopar's Direct Connection performance parts program for substantial power gains. The "Purple Shaft" (hydraulic) being one of their most popular cams for higher performance street/strip engines. Of course, more aggressive solid lifter and roller cams were available. The DC program was a parts supplier for Mopar's competition program. None of the parts were available to be ordered on a production car. IOW, you could not go to the dealer and order a 440 Road Runner and specify that you wanted a particular cam from the DC program.
I didn't read the article but if anyone is claiming a post 68 production Mopar V8 came with solid lifters, they are looking at a car that someone has previously changed the lifters and, I would assume, the cam - it would be silly to replace Hyd lifters with solids on the factory cam. All you would get is additional maintenance and lower performance.
Re the 961 Norton. There was a 961 on UK Ebay a couple of weeks ago for a Buy it Now of 10 or 12k pounds (I can't remember which). What struck me was that there were several 70's Commandos listed at higher BIN prices. Whether any of the bikes sold for their BINs, I don't know.