There is always a reason behind the Regâ€™s, the trick is to know Why.
If you want to change the cast iron stack, your call, you will be able to drill a 50mm hole using a descent â€˜hole cutterâ€™. Use a â€˜strap on bossâ€™ with a â€˜push fit rubber boss adaptorâ€˜ to connect to the pipe. DO NOT seal with silicone sealant, if its fiddly use a â€˜LITTLEâ€™ silicone lubricant or a little hand cream.
You are correct about the 1.7m distance, the reason for this is that if you use a 32mm pipe, a â€˜Sâ€™ or â€˜Pâ€™ trap you stand a good chance of getting problems later with â€˜trap seal lossâ€™. Therefore of you need to run a waste pipe longer, go up a size to 40mm or use a â€˜HEPvOâ€™ valve, or both.
And as mentioned by richieplum, the fall need to be 1:40.