Does it have to be be a lister LR1, as you are restricting your choice significantly.
Listers also made LD, SL, SR, and ST engines in single cylinder format. Whilst there are differences the external dimensions are near identical, as they all have the same mounting dimensions for bolting the engine down, and also share the same crankshaft height, so from this perspective are fully interchangable.
From a weight perspective there is also little difference to choose between these models.
Its a shame your not in the UK, as these later engines still live in the shadows of the Lister CS, and this is reflected in the asking price. For instance last month I went to the local rubbish dump and came back with a lister ST1A, which cost me 10 GPB. It needed a few bits to make it run nice, but its now my spare sawbench engine, which presently has a Lister SR1 engine on it.
These engines are much more robust than the CS, and will take alot of punishment. Or if you look after them they will be as reliable as any CS. If you know where to go for genuine OEM spare parts, they are also freely available.