Experimental category, maintenance, parts, and continuedairworthiness requirements

As the owner of a "new" 1985 DG-400, registered Experimental Exhibition/Racing, I'm trying to get a handle on what the FARs require. I have a mercifully short, ten-point set of Operating Limitations issued in 1989. Paragraph 4 states "The cognizant FAA Flight Standards District Office must be notified prior to flying this aircraft after incorporating a major change as defined by FAR 21.93." Paragraph 6 requires an annual condition inspection in accordance with Appendix D, Part 43; Paragraph 7 requires that an A&P or official repair station perform the annual; and 9 requires that the condition inspections be recorded in the aircraft maintenance records. The other paragraphs relate to general operating rules of FAR 91; placards and markings required by 91.31; acrobatics; no glider towing; day VFR only; and no flight over densely populated areas other than for takeoffs and landings.

There is no language requiring that maintenance and repairs be done in accordance with the approved maintenance manual or official requirements for continued airworthiness. There is no reference at all to the POH, aircraft manual, or maintenance manual.

Questions a

Can I do work beyond preventive maintenance specified in 43.3, without having this work supervised by an A&P?

Major changes require notification of the FSDO, so this means that the glider will remain essentially as it was delivered to the US. But, it does not have to conform to the German type certificate, as it would if it had a Standard AW certification?

If that is true, must the glider conform to DG's requirements for continued airworthiness? For example, those requirements state that the Rotax 505 must be totally overhauled at 300 hours, and that the seatbelt webbing must be replaced every 12 years. Is this legally required? (Leaving aside the question of whether these sorts of things are advisable.)

May parts be substituted - for instance, may I use a fuel filter other than the official DG part, if, as the person doing the maintenance, I determine that this substitution constitutes a minor change?

Thanks in advance for sharing your collective experience with these issues.