Powerdynamo/VAPE has been used by many here.  Great system. 

Same ignition either way, DC system has a different regulator/rectifier.  You can buy DC regulator/rectifier later to convert AC system to DC system but cheaper to get DC in the first place and just don't install regulator/rectifier until/unless you want to charge a battery.  Having the DC makes it easier to sell to an Enduro person if you ever decide on something else. 

Case pattern and flywheel mounting are the same as Enduro...  I think you have an RTMX?  When they went to MX360 case opening would have been slightly smaller so earlier RT Enduro flywheel OD might rub.  In 1974 Enduro got the MX case with smaller opening.  If your engine predates the MX360 that came out in about 73 with balance shaft provision you should be able to use any "enduro" 250/360/400 ignition from 1969 to 1979 at least.  Same mounting. 

Let me clarify a little better....

The 250/360/400 from 68 to 79 had basically the same stator plate and flywheel taper....  interchangeable. (some exception possibly for early 68 DT1) The only caveat is that in about 1973 Yamaha did a "redesign" of the engine starting with the MX versions...  The case opening was made slightly smaller so an earlier full size flywheel might rub. 

As is Yamaha tradition, in 1974 the Enduro models got the redesigned engine.  VAPE will fit them all.  With factory ignitions a 74-on will fit all.  A factory pre-74 outer rotor (Enduro) flywheel will likely rub if you try to use it on 74-on Enduro or 73-on MX. 

Thanks Mark, you all here have helped so much and have given me the reassurance I needed.

The engine is yes RT2MX, or bottom end is at least, and oddly it arrived to me with what looks like RT2 or similar mag cover (just 2 securing holes) so fingers crossed all will fit.

The engine is in a Champion flat track frame, so not very likely to need a battery or see an enduro trail.

Again thanks thanks so much.