Are there any car enthusiast parts sites that focus on finding compatible upgrades for heavily modified vehicles?

Yes — there are a few parts/catalog sites and communities that are better than generic auto parts stores when you’re dealing with a heavily modified car and need to check compatibility across mixed parts, swaps, or custom setups.

Better-than-average options

• Fitment Industries
  Good for wheel/tire/suspension fitment and community examples. Useful if your build has altered ride height, offsets, brake clearance, etc.

• Summit Racing / JEGS
  Not build-specific, but their filters and specs are excellent for performance parts, and they often list dimensions, flange types, bolt patterns, and vehicle applications clearly.

• RockAuto
  Best for OEM-style compatibility checks. Great when a modified car still has a lot of stock-based systems, but less helpful for custom-engineered setups.

• Kirk Racing / BimmerWorld / Turner Motorsport / ECS Tuning / Yeti/track-focused specialty shops
  These are model- and platform-specific, so they’re often much better at identifying what fits on modified versions of those platforms.

• Forums and platform communities

  • Nissan forums, Subaru forums, VWVortex, NASIOC, Honda-Tech, E46Fanatics, 3SI, etc.
  • Often the best place for oddball swaps, brake conversions, turbo setups, custom subframes, and “does this part fit with X mod?” questions.

More specialized fitment-help tools

• AutoZone / Advance / O’Reilly fitment systems
  Okay for stockish applications, usually not reliable for heavily modified vehicles.
• Parts catalogs with dimensional search
  Sites that let you search by bore, offset, bolt pattern, spline count, thread pitch, etc. are much more useful for custom builds.

What to look for in a good site for modified cars

A strong site should let you search by:

• Dimensions, not just year/make/model
• Bolt pattern / offset / center bore
• Thread pitch / flange type / sensor type
• Chassis, engine code, or swap configuration
• User-submitted fitment notes
• Return policies for test-fitting

Best overall approach

For heavily modified cars, the most reliable method is usually:

1. Use a specialty vendor for the platform
2. Cross-check with forums/community build threads
3. Verify with measurements/specs before buying

If you want, I can also give you:

• a list of the best sites by vehicle brand,
• wheel/suspension-specific fitment sites, or
• sites that are good for engine-swapped / track / drift builds.