The year is hardly relevent, they're all old and if that crank is 32000k old you would be less that wise to bolt a 300 kit to it.
The MC18 was discontinued in 1989, you can tell from it's shape to within a year R2j is 1988, R5k is 1989.

All NSR250's were Japan only, hence not elegible (until recent changes to the rules) to even be raced in the UK so there is zero tuning expertise and very little specialist mechanical knowledge outside of this forum
My local dealer whilst helpfull, friendly and willing to order any part number I'll give him has seen one MC28 ever (mine) and never had another NSR in the workshop. As another example I've been to reputable a 2 stroke specialist dyno and he's not known which way to cable the powervalves up on one.
Hardly the guy you want wielding spanners near your bike.
The problem is in the UK (if not Oz) the RGV250 was very popular, both it and the Aprilia RS250 were imported here officially and the tendancy is for "2 stroke specialists" here to actually be 90* RGV engine specialists. It may be different where you are but be sure.

Does the bike run?

And please, don't take this the wrong way but we try to avoid txt speak on the forum (wot, sum, ova) and slang as there are a lot of people here whose first language is not English and it renders the forum impossible to read for them, even the biggest correctly spelt English word can be looked up.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread