The MC11 is an NS250R, and a completely different motor design to the NSR250. All the NSR250 motors are based on the same crankcases, but the 1987 MC16 is solid mounted, and in the real world, probably unlikely to be chosen as a candidate for a 500 conversion.
The MC18 Mk1 and Mk2, MC21, and MC28 crankcases are all effectively the same externally, and all interchangeable. That means any NSR from 1988 onwards will accept any other years' motor.
The MC21 and MC28 are the most likely models that would be chosen to modify, as they have the greatest space between the frame rails. I have a couple of frames here and I'll measure one up for you in a moment.
The 88/89 MC18 and 90~ MC21/28 frames are based on the HRC RS250 NF5 frames of similar years. The casting isn't as heavy on the MC21/28 as it is on the RS, but the MC18 is pretty close. The MC18 is about 40mm narrower than the MC21 frame though. The front engine mounts on MC18 HRC frame were braced like the RS, but the mounts on the HRC frame for the MC21 are not, as the main frame extrusions are stiffer than the MC18. Again, the increased stiffness making the MC21/MC28 a better candidate for modification.
The RS250 front end fits the NSR very easily. It just needs a custom stem. I have a 1991 USD HRC NF5 front-end on mine now, and previously a 1990 RWU NF5 front-end on it. Both work exceptionally well on the NSR, and will fit right across the model range. NF5 RWU forks will slot right into NSR triples (both 41mm) if you don't want to use the alloy RS lower and also wish to keep the ignition switch in the OEM position. HRC magnesium Magtek wheels are pretty much interchangeable with NSR wheels across the entire range of both models. Even the RS Pro-Arm wheels from 1999 will fit the MC28, although they have to run backwards!
The MC21 Gull-Arm will certainly handle as much power as an Aprilia swingarm. Have a word with Matt at TYGA about the MC28 Pro-Arm, as to whether it's viable to run 100hp+ through one. He used to race a 75hp+ Pro-Arm'd NSR, so can speak from experience. The RS Pro-Arm is back-to-front, so it can't be substituted with a standard one.
As for choosing the Aprilia RS over the NSR, well you have one positive factor correct... junking that awful bloody Suzuki boat anchor of a motor!
Preferring the looks though, is "in the eyes of the beholder"! The early "Romboni" ones, yeah, I can see it, but the later jelly mould ones?! Pig ugly! 
I'll go grab a tape measure a moment, and let you know how wide the frames are.
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Andy.
NSR-WORLD.COM
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