Pretty hard to easy depending on your skill level, experience, time, etc. You would have to have a parts car to start with. You need everything in front of the firewall plus the shifter, pedal cluster, electronics, wiring harness, instrument cluster and ECU. That requires removing the dashboard, changing everything in front of the firewall, and the springs are now off since the diesel engine is heavier.
I would recommend against it since 00 beetle TDI are cheap enough that after your time and buying the parts car, it would have been cheaper to buy the 00 Beetle TDI outright and sell your existing car. Swaps are better for cars that you can't buy. Check out the mk4 new buying guide/owner checklist in the FAQ for some tips on what to look for.
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