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These are the 2012 T850 Rules used in England. Very close to ours and some differences.
I marked the interesting ones in RED.

M.3 ENGINES
3.1 (a).A monohull class using 870cc sports engines.
(b).Engine capacity from 750cc to 870cc inclusive. They left it at 870 but allow 90 Yamahas
3.2 (a).Only engines proposed by CRC are allowed. (Contact RYA office for list)
(b).Naturally aspirated engines only.
(c). All parts and replacement parts must be original manufacture unless
approved and listed.
(d). Engines can be re-bored, up to 870cc as per UIM rule 541. Wiseco pistons
can be used.
(e). Pistons must not be modified.
(f). Blue printing is allowed to the Homologation sheet, ports blended to a depth of 10mm.
(g). No polishing of internal channels.
(h). Repairs are allowed by welding or bonding - providing internal shapes are
maintained as standard.
(i). Reed valves as homologation but reed petal material is free.
(j). Carburettors to homologation sheet.
(k). Jets can be changed.
(l). Must use original air box.m
(m). Engine cowl must be original size and have original inlet hole size only.
(n). It is allowed to fit electric fuel pump, non of original fuel delivery system can be
removed.
(o). Ignition as homologation sheet.
(p). Ignition timings can be altered but not firing order.
(q). (i).Cylinder head one piece, welding reworking of squish allowed.
(ii).Cylinder head two piece, as above also it is allowed to modify offset squish
to that of the one piece head.
(iii).Minimum head volume including plug hole 28cc.(r).
(i). Flywheel. Standard flywheel.
(ii).Centre boss can be welded to outer disc.
(iii).Rope wheel can be removed.
(s) .(i). Gear box must have forward neutral and reverse gears.
(ii). Gear ratio is free.
(t). Exhaust must exit through propeller.
(u). (i). Propeller maximum four blades.
(ii). Through hub exhaust only.
(v). (i). Fuel Standard or Premium pump petrol only.(ii).
No additives are allowed apart from lubricating oil on UIM list.
(iii) Nitrous oxide injection is not allowed.
(w). It is permissible to tack weld the web to the pin on a Yamaha 70 crankshaft to
stop movement between the two.
(x). The Yamaha 90 engine is permitted but only in standard form, with reed petal material steel as standard.
(y) It is highly recommended that cut resistant overalls are used in all non cockpit
classes CUT SUIT SAME AS US

It will be interesting to see if their rules get closer to ours. They don’t have the 56 OMC to use and very few OMC 49’s. It is a Yamaha 70CES class. Their first option is the 90 hp Yamaha. I have not seen any pictures or race results where there has been one running in their T 850 class. The 90 yamaha does race in offshore. There is a UIM spec sheet for that motor. It is not the first time that larger displacement motors have been allowed even though there is a displacement size for the class. On another thread I posted about APBA allowing the larger Mercury motors to race in EP. Some tried them and they were not competitive with the OMC 49’s.
Our T850 70 hp yamahas haven’t done well this year. Jim has not raced, Cory has had motor trouble all year and Rick has not run many races. Based on 2012 speeds in the class, I don’t see much difference from 2011 and the boats are all competitive.
The English rules regarding the head mod and porting angles on the Yamaha are new to me. They have far more experience and knowledge about the motor than I. There must be a difference in performance or they would not have included them in the rules. Many racers must have been doing those mods and they made it a rule to keep it fair for all. Maybe the Gun and Gig can look into it to see what improvement they make. Rick is looking for a little legal improvement and I know Jim would welcome it as well.

I never said it had to be legal ...

The 90 may not be much of an improvement having the large gear case and it's a lot more difficult to make it a 15".....don't get me wrong if it's legal I'll run it lol (when my lovely wife lets me show up )