For a typical semi unit plunger engine installation, it is far easier to build the engine into the frame. Make it as light as possible, start with the basic cases and gearbox. Leave the magneto, barrel, head and primary drive off. Assemble the gearbox to the crankcases, add the right hand front mount to the crankcase, tied on loosely with a bit of wire or similar to stop it dropping off, do not bolt it on at this stage, as a bit of movement here is needed. Load this lump into the frame from the right hand side. The web in front of the camshaft is a nice carrying handle. Rotate the unit into its basic fore and aft position and lower the back end down to clear the upper gearbox mount. Move it backwards as far as possible to the frame tube, and there is usually just enough room to wiggle the unit sideways slightly to the left to allow the pesky right side front mount down past the frame lug as the engine front is lowered. Then move it to the right and up to position the mounting plate. Once the engine is in place, this plate cannot be easily added, unlike the left hand one.
Without the primary drive, barrels etc the engine is light enough to handle without a hernia. Padding the frame to prevent scrapes etc is a must.
Try all fixings for ease of use before assembly. When the engine is in place, move as appropriate to fit all the bolts and through studs loosely, then tighten them all gently to minimise localised strain, before a final tighten.
Swarfy.