Why not keep it mostly MG? These cars have modified a lot since their days (OK, in England!). If your grand-daughter needs more power (does she, really?), swap engine/trans for a modern 4 cylinder engine, Japanese, maybe... There was also an MGB V8 from the factory...
And keep the cool, sleep, unmistakable looks of the MG.
Should someone also wonder if your grand-daughter is a hot rodder at heart, is she ready and willing to live with a "hybrid", something maybe a bit difficult to maintain, with custom parts...?