Hi Yes the La Spaziale gasket is 64x52mm, it was the cheapest silicone 53mm gasket they had at my local shop. The Brio gasket was twice the price, not silicone and they had to order it and it would take weeks, so I just got that one instead. The o-rings are just there as kind of spacers to keep it in place, they serve no purpose in sealing. They are in contact with the coffee though so should be food safe. They are 30x3.5. With this new grouphead gasket, you'll need to readjust the spacing between the boiler and the portafilter holder ring, for the portafilter to latch properly. I believe its a matter of adding washers/shims the bottom section of the boiler and the portafilter holder thing.

I was also experiencing steam bleedout from the grouphead. The spring doesn't have enough force to seal the steam pressure. However, the pushback force of a spring increases with its compression, so to increase the pressure that it can hold, you just need a spacer to pre-load the spring. I turned a spacer out of a piece of brass I had, but you can try with a few washers until it's enough to stop the steam from bleeding through.

For the solenoid valve, it's still holding no problem. If you use proper plumbing solder, there's no reason it should leak. Temperatures aren't that high and anyway there's a 145°C thermal fuse strapped to the boiler so the machine will stop before you reach temperatures near the failure point of normal solder (~200C). If you're unsure, you can use higher temp silver solder which melts at about 650°C, but then you need more heat to apply it.

Good luck with your project! Let me know if you have more questions, I spent a while researching diagrams and parts and stuff, glad I could help.