Sorry buddy, nearly every writeup on this site is dependent on Imperial units, and actually cannot be simply converted. This is because we're so dependent on Imperial parts sourcing: our pipe, materials, and hardware are in inch sizes, and metric equivalents may or may not correlate. It makes it even harder that our pipe sizes are named based on imperial units, but due to differing sizing standards, not usually the same ID/OD as similarly named pipe used under a different standard (i.e.: 1/2" copper pipe, 1/2" CPVC pipe, and 1/2" PVC pipe are not the same size, despite all being plumbing pipe, PVC/CPVC being plastic pipe, and Copper/CPVC being supply pipe).
You're on your own finding out what metric parts actually go together the way we'd put them there, but we can guide you as to what should work. Here's a typical Imperial PVC pipe dimension chart (Oh yeah, there are also different schedules of PVC, which have different wall thicknesses. We usually only use schedule 40 and 'thinwall' 200-psi which is schedule 30, I think):
1-1/4 schedule 40 (or similar polycarb tube) is what we usually use for plunger tubes, and is the only size that really matters. In metric, it'd have an OD of 42.164 mm and an ID of 34.544 mm. All the other pipe and hardware sizes we use are because they nest with that and nesting won't work the same way with metric sizes as it does with imperial sizes.
All that said, SGNERF does do metric homemades. Here's one, here's a more advanced version that looks awesome IMO and I'm a little jealous it's in metric. You might be able to find appropriate rubber washers where he custom builds his.