Honda-SOHC

As you've surmised Ted, likely you'll have to "flat" it with file or block type abrasive arrangement to reduce undulations in surface.

You can use one of those house decorators rubber/Cork material blocks to facilitate abrasive materials. They'll give softer edge to help work around the area that has a boss sticking out too.

Light pressure cuts with 180 grit should bring it back to flat plane first, then successive increases in grit number to take it finer.

Use paraffin or "maintenance spray" from Asda to keep it lubricated as you work, this to prevent pickup and risk of a gouge from that being pushed across the surface. Maintenance spray is cheaper than wd40 it's how wind tunnel aluminium models were hand finished a while back.

The effect you've currently got is a little like aluminium cylinder bores that use a flexible hone to remove material around harder particals in the casting, called a "Hypereutectic" tool to particularly target this effect.

I fully understand the difficulty and why it's not possible post installation etc.

However, I like the thought process that examine things in considering solutions, whether possible of not.

From working installations (factory chemical handling) and having multiple pumps & machine arrangement, it always served to be convenient if a pump had decent access to contain lifing and environment durability needs.

Pumping from a sump, the one (inside Ted's boiler) installed in this case, we operated external pumps mounted above emergency sumps in our installation, simply with the draw side of pump piped to lowest point of that sump. It being fitted with a drop ball one way valve, then with the pump head having it's exit above it, this to effectively always keep the pump head "immersed" such that it would pull on demand to evacuate the remote sump reliably and when called by level switch.

Another, chemical automated mix machine used thus type https://www.clarksol.com/oscillating-piston-pumps/ to draw from open ended pipe in barrels of chemical concentrate, then to provide a mixed solution based on programming. 
I viewed these as "lock gate type" in which they have little Reed type rubber one way valve in flow line and located at the pump head. Self bleeding, vacuum up from barrels sitting  elbow them, ultra reliable. These little pumps used to run more or less 24hr hours at busy production period. Really easy to swap out if need to.

Couldn't the boiler be installed in raised position  ..... leaving space under to facilitate a auxiliary pump installation  ?

That's rather than a sump or underfloor void.

Not foam, but these are really good in warming boots etc before use https://www.theskierslounge.co.uk/accessories/warmers-and-dryers/thermic-electric-boot-warmers-shoe-dryers-and-boot-heaters-uk-plug__2661 there's many fancy type, but these very simple and reasonable cost items work well.

Long term skier, had this sort of heater for years to dry and warm boots overnight. Very effective too.

Only low rated heat and can always be safely left on, good for drying out shoes when wet from UK weather as well.

I'd always install one if it's the original specification and carbs were so designed.

If you've ever seen anything with a throttle that can't be competently closed then likely you'd feel the same. 

In various scenarios,  a motocross bike that had flipped and pinched the outer throttle cable was left going berserk as many tried to get it stopped.

A trike recently that someone was turning round at a bike meet, throttle stuck and rammed the rider's knee into a gate post ... to snap his femur 

A friend working as insurance assesor in motor vehicle tells me that it can invoke cover if materially contributional to accident outcome, it constitutes a modified vehicle as its considered primary safety item if fitted by the factory.
He gave example of a bike rear ended by a car at roundabout entry, punted out into traffic passing the bike to get hit by that too, no "shut" cable was fitted, legal representation unable to prove this was not a factor in the accident.

To me there's absolutely no point in not fitting one, whether you think it does any good or not. In law you're not qualified to make that decision.  There's no penalty either if the while system is correctly assembled, adjusted and maintained.

My phone is a Google Pixel 6, it has a mind if it's own. I use Flicker on my laptop & PC.

When this phone expires I'm going back to a Samsung. My IT skills are haphazard at best. I come from  Fortran IV days in 1970.

Can you do that V in your fingers, with both hands Ted ? 

My mum worked in computing when I was younger, mostly payroll system etc. When we went to see her office there was the system of punch card and a transit distribution all around the room, literally like standing inside a computer as it worked . Still have some of the punch cards as momento too.

Some internal chains have same form as kart racing final drive type chains

For which you can buy these https://www.demon-tweeks.com/demon-tweeks-kart-chain-splitter-219-pitch-kpuk219-splitter/ if they do match.