Measuring the existing pipes is obviously going to give you the sizes you need. In the USA, you won't likely find the metric sizes originally used. The 1.75" is close enough to 45mm to be used as an alternative match to original equipment. Using a caliper to measure the existing pipe diameters is the easiest way to find what you're looking for and then just match up with what is commercially available to build out your system. If you don't have access to a caliper, wrap a wire around the pipe you want to measure and then mark the wire length of wire it takes to wrap around the circumference of the pipe. Measure the marked length of wire on a ruler and then divide that number by 3.14 to get the diameter.