Was ascetic Sumedha with iddhi powers? I have heard that in the birth where Gautama Bodhisathva met Deepankara Buddha, he had many powers but he didn't use it to prepare the road the Buddha was supposed to walk across but used his own effort. Where can I find further details of this story?
It is in Thupavamsa. I doubt if there's any online version of the translation. 

The Pali version of Thupavams in the following link says, at the time the people were building the road, Sumedha was supposed to be flying in the sky. When he asked if he could also help, as people already knew that he had iddhi powers they gave him to construct a difficult part of the road.  


  Te puna divase asadisa mahādānaṃ sajjetvā dasabalassa āgamana maggaṃ
  sodhenti. Tasmiṃ kāle sumedhatāpaso attano assama padato uggantvā
  rammanagaravāsīnaṃ tesaṃ manussānaṃ uparibhāgena ākāsena gacchantā te
  haṭṭhapahaṭṭhe maggaṃ sodhente disvā kinnu kho kāraṇanti cintento
  sabbesaṃ passantānaṃyeva ākāsato oru ekamante ṭhatthāyaha te manusse
  pucchi.’Hambho kassa pana imaṃ maggaṃ sodhethā’ti te āhaṃsu bhante
  sumedha tumhe kiṃ naṃ jānātha, dīpaṅkaro nāma satthā
  paramābhisambodhiṃ patvā pavattavara dhammacakko janapada cārikaṃ
  caramāno anukkamena amhākaṃ nagaraṃ patvā sudassana mahāvihāre
  paṭivasati. Mayaṃ taṃ bhagavantaṃ nimantayimha tassa bhagavato
  āgamanamaggaṃ sodhemāti. Taṃ sutvā sumedhapaṇḍito cintesi. Buddhoti
  kho panesa ghosopi dullabho, pageva buddhuppādo tena hi mayāpi imehi
  manussehi saddhiṃ dasabalassa āgamanamaggaṃ sodhetuṃ vaṭṭatīti so te
  manusse āha, sace bho tumhe imaṃ magga buddhassa sodhetha - sayahampi
  ekaṃ okāsaṃ sampaṭicchatvā ayaṃ sumedhapaṇḍito mahiddhiko
  mahānubhāvoti jānantā dubbisodhanaṃ udakasambhinnaṃ ativisamaṃ ekaṃ
  okāsaṃ sallakkhetvā imaṃ hakāsaṃ tumhe sodhetha. Alaṅkarothāti adaṃsu.


http://tipitaka.org/romn/cscd/e1202n.nrf0.xml