I assume under the speedgoose monarchy the rates would be prohibitively expensive.
Same with motorcycles (also doesn't need regular inspections in Norway) - the riders generally have a very good reason to keep them safe for their own wellbeing.
In my state, vehicles registered as antiques are not required to have the annual safety inspection performed, and it is the owner's responsibility to keep the vehicle in safe operating condition, which is never checked by anyone.
The cars could be more safe, but we’re all mechanics by necessity. Failure also reminds us that these are all human made systems. Each can fail. Each can be fixed. In some ways being aware of the systems that make a car go (and stop) and being aware of the failure modes makes the classic car drivers more safe than their counterparts in driving appliances.
That's wildly irresponsible of you. How will you look yourself in the eye if you cause an accident like that? If you can't afford to keep a car maintained you can't afford it, period.
But in reality it'll look a lot more innocent than that, and I am sure that OP does it carefully enough.
You just gotta make sure the brakes work and that you don't do any silly things. Doing test rides and listening for sounds etc is key to figuring out what's wrong and what to improve next.
Air mass sensor failure can often be determined by unplugging the thing completely, O2 sensor failure could be the cause of your problem but if it only starts to fail at very high revs I'd check the fuel system first to make sure that there is enough flow.
Those can get clogged up pretty badly, especially in vehicles that have been standing for a while with ethanol based fuel in them, it takes forever before that gunk clears out without some work, if at all.
If it really is the O2 sensor (of which you usually have two) then I'd suspect the pre-cat one first, it runs in a much hotter environment and is more critical to the engine working properly.
Another option is the crank sensor, they may not give enough signal at a high number of revs so you start to miss if enough pulses fail (one or two in an otherwise consistent signal isn't going to cause the PLL to lose lock but if it is more erratic then it will and then revs would drop back to a regime where the sensor is still working reliably).
Good luck fixing that.
Plenty of new vehicles are a danger to others, and quite possibly far more so than an old 914. The driver of a 914 is probably going to be pretty careful about damaging it and will drive it prudently as a result, it also weighs (much) less than a modern vehicle and as a consequence will do less damage. Lack of built in safety will be yet another reason why an owner will be careful about the situations they put themselves in. They'll likely do fewer km per year in than they would in a modern car.
As a cyclist I'd rather encounter someone in their classic 914 than the local lawn service jockey in his Dodge RAM flooring it at every corner.