here is the $400 donor I bought off a 10 YO kid, I didn't really feel bad trashing this chassis. Although when I opened up the motor it was the nicest and cleanest motor I've ever opened up, especially for the age and condition of the rest of the bike.
the motor got the full treatment, every bearing, seal, even nut and bolt replaced with new OEM (yes, that is a brand new oem crank!)
nice blasted cases and jug, w/ trans w/ brand new washers, bushings and ever 2 new gears.
so... back to the conversion:
after some cuts
Y in place, measuring gap to fill... roughly 1 inch or 25mm.
tacked
a little room for the case on the right side
freshly welded top of Y
welded in after 2 1/2" dowels were installed. I rounded off the front side of the dowels so they would fit tight against the bend int eh downtube.
Y, retained stock shape of weld instead of a strait cut. the optics make it look weird, but its very nice and totally symmetrical.
front mounts fabbed out of 1/4" plate and shaved into a step to wrap around the frame. need to trim down the weld by the hole to make room for the blot head/nut.
now for the radiator f-up that evreyone wanted pics for.
I was able to bend the cross hose fittings w/o any issues, I used a deep 1/4" drive 9mm socket with an extension hooked in and just bent them forward.
the exit out the bottom was the problem. in trying to bend it, I ripped it off...
I found the radiator that came with the donor bike and chopped the tank off, then blasted it clean. I will use it to cover the hole as well as to make a proper outlet.
tried aluminum brazing and it was an epic fail, so I finally ended up MIG welding, then JB welding to make sure it didn't leak
it was laying this side down and had some drips hanging when it dried, so I lightly sanded
headstays:
used a peice of 3/16" plate, made a cardboard template, cut out the shape, then used a hammer to bend it right. the vise didn't work out so well, but my garage has some really large expansion joints, so I just laid the peices over the crack, heated, then used the pointed back of my sledge and got a nice bend w/ a few slight misses and batal scars. If I were to do it again, I'd do the same, only hold the point hammed against the peice and use another hammer to do the hammering to avoid misses.
used '03 cr250 ign.
first off the plate needs to be modified, the groove in the plate is to big around, but the right depth. so I used a 1" hole saw on my drill press to rig toigether a sort of mill and used circular modions with very little pressure to mill out the peice poking out. the oter cool thing is there is a casting mark right where the right depth is which also gives you reference for which parts to cut more from. I was in my zona and didn't get any pics. also the wires are too long and you have to pull them through the big D shaped grommet.
also w/ the '03 ign, you don't need the power valve servo hooked up to get spark, so I pulled the 5 wires from the harness including the pins in teh cdi side connector and packed the empty holes on the connector w/ silicone. this way everything is still water proof and I eliminate the extra clutter of 50 more wires and another connector.
for the '03 cdi to work you DO NEED THE CAP AND REGULATOR!
I mounted my coil where the power valve servo was. I used two 1/2" wide stips of 1/4" place with holes drilled and tepped to m6x1. the rear holes are 20mm apart and the front was 25mm apart.
ok, some additional info...
I used the cr125 59mm bushing on the right for the swingarm bolt spacing.
I used what I think were the stock cr250 hoses to go from the head to the rads (they were in a box full of hoses that could have been from any bike)
and trimmed the right side one around an inch on teh head side. I was going to just use regular 5/8" heater hose, but the coil got in the way on the left side, and I found 2 matching hoses and used them. the lower crossover hose is regular 5/8" heater hose, and the water pump inlet and outlet are just 3/4" heater hose.
the airbox did not require ANY modification, I fit the boot on the carb and just had to push a little to get the upper subframe bolt in. (just as a side note, these airboxes are notorious for leaking where the boot mounts up to the airbox. the previous owner put on one of those aftermarket aluminum braces on, but mounted it in such a way that the gasket and part of the boot were pushed in and obviously leaking, I ditched the gasket and used some silicone and took care when tightening the plate not to push to boot in when installing the lower left bolt.)
the gas tank also requires no mods. but on my bike I have some bolts stuck in it that I have to work on.
the only thing I have left is exhaust and from my initial mock up it looks like a peice of cake. the silencer will be more work, but that what the welder is for
Hopefully tonight or tomorrow I will be able to take it for a spin
your not related to Brit are you. 
