When I bought this rubber tube   I had to trim it to enable it to fit.
It needs a slight angle cut on 1 end to match the angle of the little stub pipe , plus some baseline.

personally , if I was going to filter my engine oil, I would put it (like car mfrs do) in-between the reservoir and the oil pump.
others may prefer otherwise.
however, the usual Renault 5 size screw on filters need careful allocation of space (which you may be lacking).
the filter you attached in the last link was a fuel filter not oil filter by the way

Personally , I haven't bothered with oil filters.
These classic bikes nowadays , get more fiddle time , oil changes than they ever did in the 60s.

Looking again at your pics, particularly the union at the bottom of the reservoir where very little room.
Hair drier to warm the area and pipe , feed upwards from underneath,  leaving bottom end free.
Get this on firmly and then just enough oil in the frame to dribble out of this pipe.
Already having oil can pumped oil into the feed stub under the crankcase.
This will minimise any air.

https://draganfly.co.uk/product-category/bsa/sparesparts-for-bsa-b50/air-filter-b50/

https://draganfly.co.uk/product-category/bsa/sparesparts-for-bsa-b50/gearchange/
28 is feed
25 is return (indicated by the smaller pipe to the rockers)

I don't know if these pics relate to your bike , but it may give you ideas

Certainly tight in there.
A thinner wall , maybe clear plastic , softer wall , might do.
Maybe clear thin wall fuel pipe.
Bearing in mind the original pipe is age hardened.
For the purposes of priming the oil pump etc , to see if the bike pumps oil round , just use a length of clear pipe connected to the pump inlet  pipe, fill it up , kick over to draw oil in whilst pumping oil in , start it and see if it draws in visibly , all the time topping up with fresh oil

Once you've primed everything and got it running , check the oil return to the oil tank , the rockers take their supply from this side.
The oil returned will eventually become spits and spurts , part air part oil as the scavenge gears move twice as much oil , thereby keeping the sump empty.

It'll take a while for a new engine to scavenge back to the oil tank , hence throwing in some to start with , plus the benefit of splashing about internally.
Once feed gears are filled , they'll stay filled.
The gears won't move air , they won't suck , but they'll move a liquid easily

Priming.
Remove oil feed pipe from reservoir and use an oil can pump type to fill this pipe and then continue pumping to get oil into the gears on the feed side.
Rotate engine a few times and keep pumping oil to ensure gears are filled.
Carefully reconnect feed pipe to oil tank and refill with correct amount.
Gear pumps have a habit of gravity dribbling oil through them creating wet sump , if you use your bike regularly , this keeps the engine oil in the tank
As far as scavenge side , pour a small amount into the rocker box , this will cover all the internal components to ensure oiled on start up

When you connected oil feed pipe from the reservoir to the engine , did you prime the oil pump and fill this pipe to ensure the supply gears in the pump were full of oil.
Gear pumps shift oil well but don't like air in the line

Any 4 stroke needs a certain style of starting.
Up to compression and ease over the top , then a forceful kick.
No other style will work
Both of mine will give a little kick , but this is resolved (or greatly reduced)if you follow the above routine
The correct placing of the kick start lever is vital to effect the correct swing at each stage

So you have a spark at least.
Looks fuel related.
1 of my c15s needs choke to start , the other , only a gentle tickle.

Looking at the number of pages on this subject , 7 or 8 I think, is is worth rebooting as to what you have or don't have, eg sparks and fuel and compression etc.
I have 2 C15s and each is different procedure to start

When you tickle the carb , does fuel actually dribble out of the carb visibly ?
It's not unknown for the floats and their needles to stick closed

New rods and old rockers , they have to bed in together.
Obviously , the pip is formed by the bedding in of the rocker ball into the cup , up to the point where the entire ball end is bearing on the entire cup surface.
Personally, I probably wouldn't bother , other that keeping a close eye on the valve clearances , which will be affected by this ball and cup wear.