Certainly the Mk25 was the original engine on the 34. If you remove the starter pulley etc you will get down to the main flywheel nut which will be very tight and have a self extracting action . It will loosen off and then go tight again . Further undoing will pull the flywheel off it’s taper. I say “ will” but it can be a struggle.
Back in the day Villiers supplied a special box spanner with strong tommy bar designed to be hot with a heavy hammer.
Before you go about removing the flywheel(which will involve re-setting the timing) , just remove the circular brass points cover and thoroughly clean the points , you may just be lucky if it’s been standing awhile that the points have a covering of corrosion. This can be done without removing the flywheel.
Thank you for the comments, she is now a runner! It was the timing that was off. Extremely satisfying! Allbeit a bit rough, I need to adjust the points gap and tweak the carb. Does anyone know where I can buy odds and ends like exhaust gasket and nuts to hold the exhaust and cowling on, and fuel tap on the tank? Thanks again.
If you send me a pm with your e mail address I can send an engine manual with parts list. Meetens can supply engine parts but nuts and bolts may be available from suppliers such as Namrick ,you will need to know thread etc.