A 50 hp 4 stroke is neither a good match for a Mark 2 futura (which should have a short shaft and nothing heavier than a 2 stroke 50 hp), nor for a light duty 3.8 m RIB such as a Yachtline.
Regarding the futura, there is a reason that the transom needs regluing after only 10 years! If you spend the money to have it re-glued, then slap the same motor on the boat the transom will separate even sooner than it did the first time.
Of the 3 items, no 2 will combine to make an acceptable boat & motor combination. One of those 3 items you have no investment in yet. That would be the first thing I would back away from (the 3.8. Yachtline) at least until you have firmly decided what you want to do with the other 2 items which you already have an investment in. Of the 2 items which are in your posession, one is pristine, the other needs a significant further investment in repairs to make it seaworthy, plus the purchase of an appropriate motor (which you do not currently have) to make it operational.
In my opinion, what needs to be assesed are the costs of 3 different scenarios:
1. The cost of repairing the futura, plus/minus the cost (or profit) for trading in your motor for a motor (in at least as good a condition) that is suitable for the Futura.
2. The cost of selling the futura (either for whatever you can get for it in it's current condition or what it would be worth after repairs) and applying that money to another boat (not the 3.8 m yachtline!) that is a proper match to your existing motor.
3. Selling both your boat (either as is or repaired) and motor, then applying that profit toward the purchase of the yachtline you have been considering plus tthe purchase of a motor that suits the yachtline.
Of the above 3 scenarios, #3 seems far and away the least practical.