So 88 1/2 pound is not that much. (40kg) not that much. I was expecting 188Lbs the way you was talking. lol.
So a cam belt drive is a gearbox with a belt. right.
As over £40 a pop for a drive unit. it would be cheaper and easier to put a slower motor in.
You should be able to buy those motors still. You can get them over here anyway.
https://modelskibet.dk/shop/335-el-motorer/6176-speed-600-72-v/
If they are the right ones that is. £20 a pop.... but then you still have a fast motor. I am sure you could find them in the UK.
Try here:
https://www.motorsandrotors.co/index.php?main_page=product_info&cPath=139_274&products_id=6259
I would go for something like these.
https://www.modelsport.co.uk/product/lrp-truck-puller-3-br-motor-6300rpm-12v-37685
They cost about 35 quid, but you wouldn't need the reduction box.... They can turn a 130mm screw! and they are the recommended motor for R/C tug boats.
I would say that they would push Tiger more than quick enough.
If I may say I think the props are a bit small at just over an inch. I was expecting them to be at least 2inch. Is that the radius you have given or the diametre?
It looks like there is not a lot of room for much bigger props though.
I hope that you find some of this useful. Perhaps a talk with the chap in the Modelsport link could help clear things up.
Say hi to Jen.