From your questions that you asked I think you might be getting a few things confused. A airworthiness Cert last the for the life of the airplane until "surrendered, suspended or revoked" Registrations is a yearly thing that you do through the state. If you call the FAA they will mail you a disc with all the paperwork submitted to the FAA. This would hopefully include a copy of your airworthiness cert, operating limitations, original 8130 form, previous owners, ect. You are not building you are repairing even if you do a complete rebuild on the wings. Building only applies to the initial build, other than that the plane remains in an airworthy condition until any discrepancy makes it un-airworthy. Once that discrepancy is fixed it is back in an airworthy condition. under the experimental rule these repairs and or mods can be preformed by the owner. The catch to all this is, is that an A&P (unless you are the builder) has to do an annual conditional inspection. That is why it is good to keep him involved in the process (prior to covering anything). If you change the engine out possible a new 8130-6 may need to be filled out. this is because the plane was phase tested and a airworthiness cert was issued with a different engine. The rule for the engines is any increase in HP by 20% or change from type such as piston to turbine. I would talk to your FAA PMI from you local FAA FISDO to clarify. He should also be able to tell you what manual its in. I believe it in technical order 8130.2. After completion you will need to go back into phase 1 testing, which require a minimum of 5 HR of test flight testing stall speed, takeoff speed ect. If you have the original log books it should have all the original info about this in it. A simple logbook entry "Removed engine ____ serial number___ and installed engine____ serial number ___ Aircraft in Phase 1operations" needs to be made. After testing the person who test flew the plane will make a logbook entry " I certify that the prescribed test hours have been completed and the aircraft is controllable throughout its normal range of speeds and throughout all maneuvers to be executed, has no hazardous operating characteristics or design feature and is safe for operation. Test flight completed under the following conditions: Max operation weight____, Max airspeed___, Max stall speed____. Aircraft in Phase 2 operations" You will need to work with your local FAA PMI to define your test area and to write your new operation limitations to define this test area. Sounds a lot more complex than it really is. I just got done doing this. It really was as simple as a Phone call to the FAA and a letter stating my test area and a new 8130-6. He generated me a new operating limitations sheet and off I went. I also made some major mods to our model 2. Installed Grove spring gear, model 4 nose gear, increased the vertical and rudder to the model 4 dimensions, installed a Rotax 912 and increased our gross weight to 1100 pound. All only requiring a logbook entry (I am also an A&P). Hope this info helps. Sounds like you got a great deal. If I were you I would take it.