When you say 90% complete, do you mean all the parts are there or it is 90% assembled/close to flying?

I think a big factor would be when the work was done, how well it was done, and how it has all been stored.

Full disclosure I have not built one yet. I have spent alot of time around old cars though and if things do not get used, they have big problems. If an engine has been sitting for years unused, you may need to go through it, especially things like seals.

I am not saying skip it, just be ready for the need to work out bugs.

Good luck!!