I took a sparkplug and knocked the guts out, then tok a tire valve to a grinder and ground the rubber down enough so I could push it in..

put the cylinder to TDC wit both valves closed, compress the air, see if the air seems to leak back past the valves.. You can try a compression test too.

you might find one bore wont hold comression and then the engine needs to come apart. I bored my 122's B18 to a B20, worked out fine.

if the valves are floating you can hear it.. maybe you can check how far the valves lift by checking with a dial indicator sitting on the rocker.

the rockers wear a dish shape underneath so if you set the valves and there is that dish shape its hard to get the adjustment right.

mine had valve float and blew a cam after rebuilding it, It had all new springs and was in good shape, what seemed ot happen was the grinding changed the distance between the spring lands and the valve stem keeper.. addig some shims under the springs fixed that, I just checked with paper strips to make sure I wasnt; coil binding them by adding too much..
not to scientific but it stopped the valve float issues.

122S, but similar stuff.. I used the bigger valve head and polished the ports , balanced the works.. rebuilt the carbs and put in jets for a 2L went to a D cam. added more power without completely destroying the OEM look , a volvo nut will notice the head is differnt, all alse looks pretty original

I used a distributor from a 74 with it's hall effect sensors and spark box. flipped the vacuum retard to be a vacuum advance as well. It still uses the the OEM springs and mechanical advance. seemed to work out ok. I didnt think the OEM distributor had enough advance but someone smarter could probably calculate all that.

i just happened to have the distributor all apart and noted that vacuum advance could be installed two different ways. I think maybe the Automatic used it to retard the timing. Its been going 30 years with no adjutment no pinking, no hard starting , all seems fine anyway..
i did spring for the stainless steel valve seats andhad an engine shop install them and do a proper 3 angle grind. Id installed new valve guides, they knurled them and then reamed them to keeep more oil inside the guides.. before you install new guides I'd see if they can do that to your old guides because the knurling shrinks the hole size anyway.. if you go into all that.

I never mucked withthe valve timing but i di put a new fiber gear, the metal center can separate from the fiber gear, I had that once and it made a lot of noise rattling aginst he inside of the aluminum case..

I updated the rear main oil seal.,machined out the alunimum holder to take a modern seal.. think it had a felt seal originally.

the balancing involved all the rods which they balanced end to end and against each other , and resized the big end in case it went egg shaped. I think I used lighter rods from the original motor and the crank from a 74.. that is reciprocating weight. they balanced the crank with the flywheel. I did not lighten my flywheel, I'm not racing it and enjoy it being a bit heavier. I can rev it up by doubleclutching and downshifting and and it has lots of kinetic energy so acts like a kids toy where you wind it up and let it go ;-)

all that sure livened it up a lot. my only big mistake was jsut realizing the springs needed about a .020 or maybe 040" extra shim, that was because the alves were in the head a little bit deeper and caused valve float..

some use double springs then I think you need to machine out the spring lands for a larger diameter.

I put in overdrive but it was geared so high so I changed the rear end ratio to be more like a 123, I didn't need a 5th gear so high that it could go a crazy high speed all day long..

the worn old car had a lot of backlash and it made driving it uncomfortable at low speeds, kind of a jerky action.. when I changed the crown and pinion, I also replaced the rear axle gears, it helped.

maybe if you lived in the prairies the super tall gears would be ok.

i kept the peashooter exhaust, its probably got lots of backpressure.. a 2" exhaust would probably make a difference.
the carbs make more noise with all the sucking noise, than than peashooter exhaust does.

kids these days do the opposite, they take a stock car and add an oversized muffler, making it sound like it's faster than it is. I'd rather have a sleeper.. I can race old ladies in their brand new honda's without them even knowing they are being raced.