You need to download the challenge and build from that. Replacing or removing literally every part, including the cockpit, is totally fine so long as you work from that starting point. However, because that's a bit of a pain, what I usually do is:
Build my entry just like I would build any other plane, without downloading the challenge
Save the whole thing, except for the cockpit, as a subassembly
@Samwise oh you're the guy that fixed the details on my X-wing! Wow, I'm impressed you remember me hahaha
@Skua
Thanks again skua
You need to download the challenge and build from that. Replacing or removing literally every part, including the cockpit, is totally fine so long as you work from that starting point. However, because that's a bit of a pain, what I usually do is:
Build my entry just like I would build any other plane, without downloading the challenge
Save the whole thing, except for the cockpit, as a subassembly
Download the challenge
Strip away everything except the cockpit
Drop the subassembly in
And now it can be uploaded as a successor.