Hi Xander 2121,
Do you just want dirt tyres for your existing wheels or do you want a separate set of wheel and tyres?
There are dual sport tyres that fit 17" rims.
If you what to go 21/18 inch wheel then read below.
I don't know the answer to this with out a bit of research.
I would say anything is possible with the right combinations of parts, determination and money.
By the looks of a 230M I think the 21 will fit at the front.
You would have to check out a typical 18" dirt tyre to see if it fits in the swing arm and the axle will still have a spot to go to.
Some times they fit the arm but the axle has to be so far back it has no slot to fit in.
It also depends how firm the 230M suspension is so it doesn't bottom out guard to tyre.
Well the 230M has 17 inch rims front and rear as you stated.
You need to measure axle sizes and widths to see if they are standard CRf230 or they were made larger.
Suggest you look in the OEM parts section here
http://partsfinder.onlinemicrofiche.com ... finder.htm
You can then compare part numbers of axles bearings and wheel sets between the different models that might become donor bikes.
You do have to consider whether the 250/450 hubs are wider and if changing spacers around or making new can get it to fit.
Check out brake disc diameters and offset from hub and whether the callipers line up.
Can be as simple as going to a dealer and measuring stock bikes to get an idea.
If the tyres can fit then count the number of spokes on the front and rear of your existing wheels and compare to 21/18 wheels.
If you are lucky they might be the same number and you could possibly lace the larger rims to your existing hubs.
Having the right spokes maybe an issue as there are a number of different ends to lock into the hubs.
The wheel manufactures might be able to help here as I have had spokes made up to fit different hub wheel combinations in the past.
I know people have put Super Moto tyres on dirt bikes as well as lowered them to get it to handle.
This has been done in the same family of bikes though, making say a 450R into a 450SM.
Can't say I have seen it go the other way.
Hope that gives you some idea wear to start, and if you figure it out please post up what you did as I am sure others would be interested.