Still plan on using a Triumph Rocket engine in the trike.

This engine is mounted longitudinally in the bike and the final drive is a shaft.

I want to keep the trike wheelbase relatively short and run a ridiculously wide rear tyre, so for those reasons I'm thinking of
turning the engine through 90 degrees anti-clockwise and attaching a sprocket to the output shaft.

So my first Q... is there a great deal of difference in the forces experienced by an engine driving an inline shaft or a chain at 90 degrees? And if so, what would be likely to go 'wrong' if one was used as the other (bearings, cases, etc.)?

I appreciate this is a how-long-is-a-piece-of-string issue, but all opinions welcome!

Be aware of engine rotation or you could end up with 6 speeds in reverse
Denis.

Got that sussed Denis... 90 degrees anti is the way forward (literally in this case!).

My gut feeling is yes, that an engine designed for chain drive would have much greater strength and bearing capacity for sideways loads on the output that a shaft design

I would expect a very short life on the Shaft Oil Seal.

As a minimum I think you would need a short output shaft (carrying a sprocket) with an outer bearing support to ease the sideways loads imposed on the engine by the chain drive. A bearing carrier either side the sprocket would be better to remove the sideways loads from the engine completely, a big job but technically simple.
I think the rocket 3 engine is quite long (wide) so I would expect a bigger problem could be the position of the drive relative to the rear axle especially if you want wide rear wheels, it would be well over to one side.