Not to discourage you from your plan, but I personally wouldn't give my money to a big name mill. Their employees rarely take the same care and quality that the name who built the company would take. ESR is a good example of this. I'm not saying all their work is no good, but I can say the stuff I had done there was. I've seen better luck with reputable smaller builders who are active in their own shop.
Also, if you plan on going "big" you may want to consider a non-oem cylinder. The porting configuration on the stock 250r's though not bad and can definetely be made to run hard, were not designed for massive output. Sometimes a bolt on with some minor porting changes is an easier way to go. You can always sell a stock 88 cylinder for a decent dollar if it's untouched.
Also, know that stock TRX configurations are easy to make big improvememts on without massive, permanent modifications. Honda made the TRX pretty docile by design so a lot of quick power can be had pretty easily without a lot of expertise and risk that the build won't be what you want and the changes irreversible.
Above all, remember that big power comes from more than just what is done to the cylinder. Money should also be spent on intake, exhaust and definitely the head.
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All250R - For the development and preservation of 2stroke racing technology
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Mods on a 1988 Honda 250r 
Linear Mode
