This is not designed to fit any specific bike. This fender is typically a last resort for bikes that do not have plastic available any longer. You may be able to mount this up on your 1986 Honda XR250R but it will not be an exact fit so some modification may be required. From what I can tell the original fender for your bike is still available from honda, you can find the OEM diagram with that rear fender listed here: http://www.motosport.com/dirtbike/oem-parts/HONDA/1986/XR250R/REAR-FENDER?sku=80100-MN1-670ZB
Will this fit a 2005 KDX 200 I don't mind if I have to do a little modification but is that even realistic with this type of fender? I don't want to pay the $65.00 for an oem one so I thought I would give this a shot/.
This is not designed to fit any specific bike. This fender is typically a last resort for bikes that do not have plastic available any longer. You can make it fit but it will not be a correct fit for the KDX200.
These are a universal type fender, so they typically don't just "bolt up". You may want to check out the OEM Parts Diagram page for reference of the stock rear fender to compare the two and see if it will work for you. Some trimming or drilling may be necessary.