My dads next door neighbour also has a mk5(?) tina, a V reg one, that is his everyday car running a 32/36 and he's still on points!!
Your idea sounds the best option, though I doubt very much the tank would fit ?, pipes could easily be welded to a tina tank though, only two needed one around 3/4" to feed the injection pump, then a return, for which you could use your original feed pipe.
Just be sure to get the engine loom with the donor car, there's only around 4-6 wires to hook into the tina's wiring to get going.
He's also put a 5 speed box in his car, it split another hose at the weekend, 2nd one in about 3 weeks, though all original and 31 years old!!
tabetha