streetable answer
You've tried to ask this same question several ways in several areas of the forum.
I would like to discourage you in the strongest possible terms.
For what you're asking, you want to get a bike from the same company from about the same time period and nearly same displacement.
For example, get an SRX250 and it will have a magical VIN on it and it will already be street legal.
It is then completely illegal to transfer the VIN to another Yamaha that might be 250ccs. The EPA never approved the TZ250 for the street here.
So don't do this.
I had a desire to do this a long time ago but fortunately it passed. Why do you want to have magnesium rims out on the street? Race stuff in addition to being FAST is barely strong enough to survive racing. A racing wheel will always be prone to bending more than a street wheel. Will you run slicks? I hear CHP will impound your bike and fine you more than $300 if they catch you with something like this. Cool!
Unless you have a specific idea of what you want to do and realize the trade-offs don't even try. To have never been on a torture-rack RS125 and assume you'll be fine on it isn't wise (yes, I know, you're tougher than me). I'm 6' and it's painful to race it every month. I take an Alleve before I hop on it.
Do you want a powerful thumper in a good chassis? Do you want a full grey-market bike? Do you want a 2-stroke? The answers for what to do will all be radically different.
I put a 500 thumper in my old SRX250 and it was "fun". But "fun" included having gearing something like 16 x 36 and a first gear moon-high to enable running at highway speeds without grenading or shaking apart. Dirt bike transmissions belong on dirt bikes. Unless you have a talent for being able to re-engineer the transmission gears to make it a wider shifter it will always be like this.
If you see street legal NSRs and think you can do it with another bike rememeber the key difference is the NSR is a street legal bike in many parts of the world. Thus you can buy a light for it. Also, it has an electrical system which is compatible. Are you going to put in a 12V battery for your lighting system and only charge it at home because there's no lighting coil on an RS125 or many of the dirt bike engines? Not a good idea.
Yes, I'm old and boring, but I do understand the urge to have something "different". A LOT goes into re-engineering a bike that you'll find out the hard way. Just recognize your finished product will never be anything close to "factory" by the time you're done. You'll always be explaining to your buds that you have to tickle the carbs JUST RIGHT so it starts, or that it fries its batteries every month or it can only go 80 miles on a full tank or that it balks with bad carburation at low revs.
Good luck and get something stock for the sake of your own sanity!
race bike bad seating position..... my other bike i'm considering is an NSR50, and you really cant make an arguement that an RS125 is less comfortable than a little ass NSR... can u?
Mark 
