Anas Sarwar has claimed there was nothing “under the bonnet” when he took over “hollowed out” Scottish Labour last year.
He said he expected to find an engine that was "tired" and "dilapidated", but instead it was empty.
The Glasgow MSP also said the scale of the challenge he faces is greater than the issues faced by past leaders like Neil Kinnock and Tony Blair.
Sarwar will tomorrow address his first proper conference as Scottish Labour leader since taking over from Richard Leonard.
As revealed by the Record, he will confirm plans to scrap the historic red rose emblem and replace it with a thistle.
The rebranding is one element of a wider programme of reform he hopes will turn around the fortunes of a party trailing in third place.
Speaking about the new logo for the first time, Sarwar told the Record: “I'm not for a second suggesting, and I don't want anyone to think this, that somehow you change the logo and it changes the way a single person votes.
“So this is not about saying 'we've changed a logo, this is going to have electoral benefit for us'.”
He said one part of his change agenda is about “modernising the Labour Party and bringing it into the 21st century - 22 years into the 21st century”.
He said this involves branding, how Scottish Labour delivers its messages and being a “digital first organisation”.
The Glasgow MSP said the more “fundamental part” is changing the culture and “mindset”, such as ending the “culture of defeatism” in his party and the idea Scottish Labour just manages decline.
Asked about the state of the party when he took over, he said:
“I expected to look under the bonnet and to find an engine that was tired, dilapidated but actually still had a bit of verve. I looked under the bonnet and it was empty.
“So we were absolutely hollowed out as a political party and as an organisation."
He said: “An unbelievable amount of my time has gone in to rebuild us as an organisation to be fit for purpose, and to be a political party worthy of its name.”
“I think we've rebuilt the organisation so it's fit for purpose. I think we managed to do that. The next phase now is to turn all of that into electability, and that is about fresh ideas and fresh thinking, that means the right policy approach.”
In the 80s and 90s, Kinnock and Blair reformed Labour across the UK in a way that led to three general election victories.
Asked if his own rebuild was similar, he said: "I think it's absolutely of that scale, if not even bigger.
“There is no quick fix. There is no silver bullet. We have had 20 years of decline in Scotland and that requires us to reboot the Labour Party.”
“I want people to look at the Labour Party, realise it has changed, realise it's determined to change even more and rooted in the realities facing Scotland today.”
His problem is votes in Scottish elections are tied to views on the constitution, with the SNP popular among pro-independence supporters and the Tories doing well among staunch backers of the Union.
Scottish Labour, by contrast, has haemorrhaged votes to both parties since the 2014 referendum.
Sarwar said: ”Why accept the frame of politics the way it is just know?”
But what if it doesn't change?
He replied: “Why can't it change? Where is the will for it to change? There has to be a will for it to change and there have to be people willing to do the hard work for it to change.
“The Labour Party has never accepted the world the way it is. Why are we accepting the world the way it is? Why are we accepting our politics the way it is? I honestly think we can change it.”
SNP MSP Rona Mackay said: “Instead of rearranging flowers in a logo, Anas Sarwar needs to wake up and smell the roses.
“While he concentrates on inconsequential nonsense, he shirks away from the single biggest issue driving his party's failure in Scotland - the constitution.
"Until he ends his Trump-like denial of democracy by accepting Scotland’s right to choose self-governance in an independence referendum, his party will continue to slide into irrelevance."
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