The rulebook states, M class requires the engine be same brand as chassis. It does not require the engine be from a particular model or type, just a motorcycle engine from the same brand. Factory race bikes, complete, are limited to A. For example, there are folks still running various factory flat-track/roadrace Harley engines in street based chassis, with some very good M records to show for their work.
You have to make the engine fit without cutting the main frame spars or backbone, so it might take a different Yamaha with some room in the chassis to accept that TZ type or clone engine. Some of the later FZR bikes could be very good with an engine swap. I like to think of M bikes as the Gas Coupe or Comp Coupes, and A bikes as the Lakesters. 
Truth is, its now possible to build more power than the factory racers had, but you'll not succeed with stock street bike cooling. That is one of the advantages of the factory race stuff; cooling that could go the distance. Also, you can now use better streamlining than those race bikes had, such as an A type fairing and front fender.
Fun stuff, and a great challenge to fit it all into a street chassis. You should do it!
Regards, JimL
I was just asking the question because I wanted to understand what the benchmark was based upon when making a comparison.
The Modified Class is intended for “modified” production models and not purpose-built racing bikes.
This class includes all On Road, On-Off Road and Off Road only models and limited production models
(more than 50).
This class does not include factory produced road racing or any other “works” racing models.That's the current rules, so I could not use a RS250 Honda or a TZ250 Yamaha in Modified, now, according to this.
For me in the M and A smaller capacity classes, a 2 Stroke is still the weapon of choice, even though development on Road based engines stopped in the mid 90's.
250 Road Race engines however continued until early 2000's and 125's into 2010, but getting a Works Aprilia engine??.
There is a marked difference between
potential output of a TZ or a RS Road Race engines 85-95 HP and a RGV, NSR, KR1s or a TZR Road engine that max out at 70-75 HP.
So for A classes the RS or TZ is the go.
We were lucky down here in OZ as the early MOD 250 records were Open or set by 4 Stroke/4cyl CBR250rr Honda's that are still eligible, and in recent years a few of us are set on raising the 250 Bar using Smokers
The Kawasaki KR1s has Production records at Bonneville 139 and El Mirage 141 which is very impressive and realistically are our initial benchmarks.
An RGV based bike ran 131 mph this year, but has a lot more to come I'd say. My RGV APS class bike had a few issues so it is still an unknown.
I will also be having a crack at the M-G 250 on a naked NSR250 Honda next year and the other two RGV's based bikes will be back in PROD and MOD.
In the end it's about having a go and running what you can, whatever that is. It will be an interesting next few seasons in 250 world with our Smokers and a certain Blown CBR Honda, onward and upward I say
Pete
