Firstly, there are two con rod lengths for the B33. Early B33 engines share a con rod with the B31 which is 7 3/8" between centres...
Later B33s had the rod shortened by 1/2" so have a measurement of 6 7/8" between centres.
The M20 and M21 have a con rod that is longer than either of the OHV motors at 8 1/4" between centres.

There is also a difference in the timing side mainshafts...The side valve motors have a longer timing side shaft and the boss on the flywheel face is also shallower on that side...

Further, all OHV flywheels are steel, some M20 flywheels are cast iron (but most are steel) and most M21 flywheels are cast iron with the steel version being rarer...

This can be relevant if the flywheels are to be lightened (or revved) as the steel wheels are really required in that case...

This would be a long post if I went into the specific details of what has to be done to allow the use of the side valve flywheels in a B33 but both M20 and M21 wheels can be used with appropriate modifications. In either case one of the OHV con rods is needed, the early one for an engine fitted with M20 flywheels and a later short one if M21 flywheels are used...

I have built multiple OHV engines with the M20 flywheels but only two with the M21 flywheels...To use those requires a fair bit of work to get everything to fit, including relining the barrel and boring the crank case mouth out if the maximum capacity is required (720cc..90 bore by 112 stroke)...

Dimensionally in that case the engine is being taken more or less to the limit. There is also the less radical option of retaining the B33s 85mm bore to give a final capacity of 635cc with the M21 flywheels....

I did it to see what the result was like but actually consider the 94mm stroke x 90mm bore piston (600cc) engine with suitably modified flywheels and other tuning mods to be the best package by far for practical use..I use modified 350 heads on those as well but that's another story!...

The actual mods required to fit the flywheels aren't too complex...any good engine machine shop or toolroom should be able to carry out the work...The crank may also need to be rebalanced depending on the engine characteristics desired...

In fact, the exact specification of the flywheels and the rest of the engine really rather depends on what you want to do with it and deciding that is actually the place to start.