That's your problem right there.
Wing variants only come in certain-rank sizes, so they get offered to you as-is. But solo missions scale to rank.
And yes, wing assassinations will be virtually impossible without a murderboat... but they're simply not worth doing generally.
but i can blitz the wing missions on low res sites so i can get the 20-25 mill payout while the solo missions on my level wont even pay a fraction of this, for me, its still these are the only ones worth doing. maybe once i will have elite missions in solo, i will have those, but for now, i have a 900k offer for a solo massacre... im not going to waste my time with that