Is this OK?

Aslinda, bir adada kaldık. Sadece bizi. Çok büyük bir parkta oldık.

Adaya kano ile gezdik. Zor iş. Sitemiz bulmak biraz zordu. Biraz kaybolmuştuk. Haritadan kolay olmuş.

There are some mistakes in your text. 

Aslında bir adada kaldık. Sadece biz(bizi). Çok büyük bir parkta oldık(I do not understand what you mean).

Adayı kano ile gezdik. Zor iş. Sitemizi bulmak biraz zordu. Biraz kaybolmuştuk. Haritadan kolay olmuş(I do not understand).

Thanks!

Çok büyük bir parkta oldık(I do not understand what you mean).

I was hoping to use olmak rather than kalmak - We were in a very big park.

Haritadan kolay olmuş(I do not understand).

I wanted to use this tense: duyulan geçmiş zaman (heard, indirect, reported or inferential past tense) - From the map it seemed easy.

You can not use ´´olmak´´ instead of ´´kalmak´´ here. ´Çok büyük bir parkta kaldık´ is correct. To keep closer to the original, you can say ´Çok büyük bir parktaydık.´ Both are possible.

´Haritadan kolay gözükmüştü.´´ (This is correct and it means you saw the map.)

If you want to use ´´duyulan geçmiş zaman´´, you can say ´´Haritadan kolay gözükmüş.´´ 

But this form does not go with your text. Because if you use this, I understand that you did not see the map. People around you saw it, and they told you it seemed easy from the map. Why is it not good? Because before the sentence, you wrote that you were lost. You were in the action. It is not reported to you. 

If you wanted to say that your friends saw the map and you did not see it, this is better;

´´Arkadaşlarım gelmeden önce haritaya bakmıştılar ve haritadan kolay gözükmüş.´´

´Aslında bir adada kaldık. Sadece biz. Çok büyük bir parkta kaldık.Adayı kano ile gezdik. Zor iş. Sitemizi bulmak biraz zordu. Biraz kaybolmuştuk. Oysa haritadan kolay gözükmüştü.´´

When I read this paragraph, it is impossible for me not to realize that it is written by a non-native speaker. I want to re-write it.

´´Aslında yalnız başımıza, adanın birindeki büyük bir parkta kaldık. Bir kano vasıtasıyla adayı gezdik ki bu pek kolay değildi. Sitemizi bulurken biraz zorlandık. Açıkçası kaybolmuştuk. Oysa haritadan kolay gözükmüştü.´´ 

Based on what I read here, I thought I could use that tense:

http://www.turkishlanguage.co.uk/pasttense.htm

Presumably he has arrived.

[possibility]

From the map, presumably it would be easy (i.e. to find).

Its pretty sure that they have left the car in the garage.

[probability]

From the map, it was pretty sure it would be easy (i.e. to find)

I think that the bus left at 6 o´clock. 

[uncertainty]

From the map, I thought it would be easy (i.e. to find)

Their explanation is not clear. They should have translated these sentences into Turkish. 

´´Its pretty sure that they have left the car in the garage.´´ does not have the same meaning as ´´Arabayı garajda bırakmışlar.´´ 

Arabayı garajda bırakmışlar = According to what I have heard they left the car in the garage.

Its pretty sure that they have left the car in the garage.= Kesinlikle arabayı garajda bırakmışlar. = Arabayı garajda bıraktıklarından eminim.

I think that the bus left at 6 o´clock.= Sanırım otobüs saat 6´da gitmiş.

You can not say ´´Otobüs saat 6´da gitmiş.´´ It is not a possibility. It means that it is reported to you.   

You thought that ´´ I can find there easily´´ after you had looked up the map. 

However, when you were sailing, you realized that it was not easy.

So, you say that ´´ After we looked up the map, we thought that probably it would be easy to find.´´

´´Haritaya baktıktan sonra, kolaylıkla bulabileceğimizi düşünmüştük.´´

´´Muhtemelen haritadan kolay gözükmüş.´´ If you say it, we understand that you did not see the map, and it is not reported to you. You just guess it. You think that ´´ They saw the map and they thought that they can find it easily.´´

thankyou