Keep it standard, thats when they are at their best, at the most a de-cat and a free flow filter.
People appreciate Valvers more when they haven’t been molested, just check service records and make sure every 5 or 6000 thousand miles you change the oil and filter, every 10 change spark plugs and gear box oil, 15 fuel filter and brake fluid and of course change the anti-freeze whenever it looks a bit s**t…