Planning a conversion with a 15hp V twin

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I have enough spare parts to just about make a BMW R series rolling chassis minus the engine now that the sidecar engine has blown up and needs replacing. Basically I was going to make a R80 cafe racer style bike our of my accumulated spare bits but now I'm suddenly minus an engine it leaves me with a second plan and try and fit the 15hp V twin diesel in it.

Does anyone have any experience with BMWs and diesels??

I've got two final drives to choose from but after a bit of spreadsheeting it gives a top speed of either ~50mph or ~45. I was expecting to aim for about 70 as top speed so I'm guessing that the available ratios are not compatible?

With R80 drive.
rpm ratio gear final rpm wheel wheel cir kph mph
3000 4.40 1st 3.00 227.3 2058.37 28.07 17.43
3000 2.86 2nd 3.00 349.7 2058.37 43.18 26.82
3000 2.07 3rd 3.00 483.1 2058.37 59.66 37.06
3000 1.67 4th 3.00 598.8 2058.37 73.95 45.93
3000 1.50 5th 3.00 666.7 2058.37 82.33 51.14

With R100 Drive.
3000 4.40 1st 3.36 202.7 2058.37 25.03 15.55
3000 2.86 2nd 3.36 311.9 2058.37 38.51 23.92
3000 2.07 3rd 3.36 430.9 2058.37 53.21 33.05
3000 1.67 4th 3.36 534.1 2058.37 65.96 40.97
3000 1.50 5th 3.36 594.6 2058.37 73.43 45.61

Tyre
110 Width
90 Profile
18 rim
655.20 Diameter
2058.37 Circumference