I just bought a crashed motorcycle from my neighbor but is not driveable because the forks need to be replaced. Does anyone know if I can replace them with forks from a different brand, maybe honda or kawassaki?
Yes. In short, you can mix and match many different brands and models, provided the dimensions are correct.
Fork Tubes Only
If you want to replace only the fork tubes, you will want to locate a new set that has the same outside fork diameter, measured where the triple tree clamps onto the forks, as well as the same fork "style", i.e. standard or Upside Down.
This will keep you from accidentally getting the wrong size. Fork tube sizes almost always reference the narrower, inner fork tube diameter. While an upside down fork might technically have a given diameter, the area where the forks clamp to the triple tree is the outer, much larger tube, rather than the narrower inner fork tube
Whole Fork Assembly:
You can replace the entire fork assembly including the triple tree, with a set from a different bike (of any manufacture), provided:
- The upper and lower headset bearings are of the same dimensions between the two sets of forks
- The fork pivot tube on the new forks is the same diameter (it will fit inside the existing head tube)
Differences in the vertical length of the head tube can usually be accounted for by moving the triple tree clamps up and down on the fork tube until the correct length of the head tube is matched, but verify ahead of time that these have enough adjustment room to accommodate any changes you need.
This site has a huge database of headset sizes, so you can easily find models that will be compatible with your current frame: https://www.allballsracing.com/index.php/forkconversion
In either case, be aware that once you replace the forks, you will probably also need to replace your front wheel and brake(s) with ones that match the new forks. You might luck out and find one or the other of your current parts fits the new forks, but that's a small chance, and even smaller that both will fit. Again, shop around for compatibility before you buy anything to see what's available.
Also in both cases, it's ideal to try and find replacement forks that match the overall length of the original forks. Switching to a longer or shorter set will affect the overall geometry of the bike, and can dramatically change the handling.
Finally, if you can't find an exact match, there are a number of retailers that sell standard size conversion kits, or that can even custom build a triple tree to fit the dimensions you need.