Sadly, Torben, I think this will prove to be a difficult modification. I did own a C260N for a few years (albeit nearly 40 years ago) and I seem to recall that the spindle was designed to take 30mm bore spindle tooling and ran at 4500 and 7000 rpm. That means it is suitable for use with a Euro spindle block, providing you have the correct guarding (Shaw - or SUVA-guard). but that it is far too slow for use with any form of router cutters (which generally require 18,000 rpm and above).
"The person who never made a mistake, never made anything" - Albert Einstein
"I too will something make, And joy in the making" - Robert Bridges, 1844~1930