If you make a decision at the start and change your mind later, that’s okay.
Plans are just a tool to keep you on track, not laws that you are legally bound to meet.
As much as it is helpful to have a guiding force to keep things focused, it is essential to be able to go with the flow when problems and required workarounds inevitably arise. As the situation evolves with your build, the plan must evolve too.
This suggests your build plan is a living document that needs to be updated when circumstances change.
If the time needed for a task blows out, update the plan with new information with dates.
If you lost some parts and need to reorder them, update the plan with dates and costs.
If you ran out of money for the engine rebuild, update the plan with new dates.
If you've realised you can complete another task while waiting for parts to arrive, update the plan and re-sequence your tasks.