Paul,
3.75x19 tyres are probably from Russia, intended for Ural motorcycles, or China and intended for CJ750’s (an old BMW/Ural copy). Both those marques are normally used as sidecar outfits.
Whilst I don’t have experience of the Russian tyres you need to figure that they’re designed primarily for sidecar use and may be less than satisfactory on a solo machine.
I do however have an unpleasant experience of riding a solo CJ750, and I can report that the original tyres were absolutely lethal on a wet road because they’re square-shouldered and stiff, and I would think they’d also be very unpleasant to use on damp grass.
So if you’re going ahead with this, please use caution.