And welcome to NSR-WORLD.COM
We will do our best to accommodate you and help with any questions, but please note that this is an English speaking forum, and you may find responses limited if you post in Spanish. We do have a few Spanish members that visit from time-to-time that may be able to help you with local resources, so you've found the best place for help!
Although an expensive option, a new MC21/MC28 crank can be machined to fit R5/6K cases, however finding someone to do it, and do it well, maybe difficult. While I have heard of cranks being machined as an assembled unit, ideally it needs to completely disassembled [which will damage the main bearings], then the webs machined to R5/6K spec, and finally reassembled [with new main bearings] and balanced. I would guess this option however, is likely to end up costing around 1200-1500 Euros. Given the increasing value of all NSRs though, this may be a worthwhile investment.
Ultimately, as Rothmans_SPud asks, it depends how bad your crank is, and if it can be rebuilt. The common failure of 89-96 cranks is the centre bearing/seal failure, and most of the time the remaining components are reusable. A crank builder will likely require a non-refundable inspection fee to disassemble your damaged crank and measure all the components. If they are within spec, an assembly fee in the UK is normally around £300-400 + main bearings + return shipping.
Note: If your crank is not suitable for rebuild (example, a big-end journal is damaged) then the crank will be returned in component parts or disposed of, depending on the company you are dealing with. Do not expect a reassembled crank to be returned if it doesn't pass inspection, but any company you end up dealing with will be able to advise specifically.
GT Performance Engineering are the best in the business, but exceptionally busy. They deal with international customers a lot though, and best contacted through Facebook: https://www.facebook.com/GT.Performance.Engineering
_________________
Andy.
NSR-WORLD.COM
Please keep all responses to Forum posts on the Forum so that others may benefit.
Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.
Check us out on Facebook! Please "like" our page!